Add alpha transparency to SVG backend

Update documentation for alpha channel

This change adds documentation about the new alpha channel parameter
when using `rgb-color`. It also renames the sections and the uses of the
function, while keeping "RGB" in the index in addition to "RGBA".

Add alpha transparency to SVG backend

Adding the alpha channel to the color commands allows for objects to be
of different opacities, leaving more flexibility in coloring. This is
only available with the SVG backend as PostScript has poor support. The
`rgb-color` command has been renamed to fit the alpha channel as
`rgba-color`, with a convert-ly rule associated. By default, the channel
is set at 100% opacity. The built-in colors have also been updated.

In addition, setcolor now outputs values from the 0-255 int range, and
not a percentage in the SVG backend.

[Ebe123, 2017-09-21 00:28:23]

Adds more flexibility to the coloring of elements in SVG files. It is
disappointing how there is no support in PS, and thus our generation of PNG.
This is useful though in my view, and at least it's available somewhere.

[Knut_Petersen_t-online.de, 2017-09-21 10:04:53]

> Message:
> Adds more flexibility to the coloring of elements in SVG files. It is
> disappointing how there is no support in PS, and thus our generation of
> PNG. This is useful though in my view, and at least it's available
> somewhere.

Postscript/Ghostscript allows to define transparency for a pdf target.

BUT: We build our scores from a lot of symbols and a few postscript functions.
Most of the time those symbols overlap. What would be the result if we would use
transparent colors for stems, noteheads, ledger lines, flags, accidentals, etc?
I think the result would be pretty useless unless your 
intention is to show how we  construct our scores. The attached lilypond source
demonstrates how to persuade lilypond to generate a pdf with transparency, and
it demonstrates the problem I mentioned.

You invested a lot of work, so the probability is high that there is a real use
for the feature. Could you be so kind to give an example to a man with a lack of
imagination?

Knut

[Ebe123, 2017-09-22 00:37:31]

On 2017/09/21 10:04:53, Knut_Petersen_t-online.de wrote:
> > Message:
> > Adds more flexibility to the coloring of elements in SVG files. It is
> > disappointing how there is no support in PS, and thus our generation of
> > PNG. This is useful though in my view, and at least it's available
> > somewhere.
> 
> Postscript/Ghostscript allows to define transparency for a pdf target.
> 
> BUT: We build our scores from a lot of symbols and a few postscript functions.
> Most of the time those symbols overlap. What would be the result if we would
use
> transparent colors for stems, noteheads, ledger lines, flags, accidentals,
etc?
> I think the result would be pretty useless unless your 
> intention is to show how we  construct our scores. The attached lilypond
source
> demonstrates how to persuade lilypond to generate a pdf with transparency, and
> it demonstrates the problem I mentioned.
> 
> You invested a lot of work, so the probability is high that there is a real
use
> for the feature. Could you be so kind to give an example to a man with a lack
of
> imagination?
> 
> Knut

Seems way less friendly than by integrating the feature with coloring
(rgb-color). This method is also more limited to PS, whereas my focus was more
on SVG. Thanks for the way though, as the integration with other backends could
be added too; I was mistaken.

As a use for this feature, it can be used to put focus on a specific place,
using a lower opacity elsewhere; more usefully, it would allow objects with
.transparent true to be added to the file, not completely omitted (caused me
some trouble). In your project for visual score playback (video), it would allow
for MuseScore-style bars (if wanted). Basically, it is more for debugging, but
cool stuff like color blending with near objects and color interaction would
also be possible through this as a first step.

This was more a grep work; very little adds the alpha functionality, most just
updates existing colors. It's possible for me to waste time on the useless
anyways. :)

[Ebe123, 2017-09-22 11:19:08]

Add to regression tests

[dak, 2017-09-22 11:34:30]

On 2017/09/22 11:19:08, Ebe123 wrote:
> Add to regression tests

I think it is a bad idea to change rgb-color here: this is a recipe for
wide-reaching incompatibilities for the sake of limited applications.  It would
make more sense to meddle with the type of the "transparent" property, making it
a boolean-or-number? for example.

If transparency is supposed to be made into a part of color, I think it should
rather be optional: see
<https://sourceforge.net/p/testlilyissues/issues/5162/#bd3f> for a proposal in
that regard.

However, I think that transparency is really something that tends to have
considerably different scope and application as color and is more often than not
applied in manners orthogonal to color, so keeping it in a separate property
might make for a lot less conflicts of use cases in the long run.

[Ebe123, 2017-09-22 12:01:54]

On 2017/09/22 11:34:30, dak wrote:
> On 2017/09/22 11:19:08, Ebe123 wrote:
> > Add to regression tests
> 
> I think it is a bad idea to change rgb-color here: this is a recipe for
> wide-reaching incompatibilities for the sake of limited applications.  It
would
> make more sense to meddle with the type of the "transparent" property, making
it
> a boolean-or-number? for example.
> 
> If transparency is supposed to be made into a part of color, I think it should
> rather be optional: see
> <https://sourceforge.net/p/testlilyissues/issues/5162/#bd3f> for a proposal in
> that regard.
> 
> However, I think that transparency is really something that tends to have
> considerably different scope and application as color and is more often than
not
> applied in manners orthogonal to color, so keeping it in a separate property
> might make for a lot less conflicts of use cases in the long run.

So [#5189] would be preferable to this. There would be way more work if done
that way. Alpha transparency is integrated with color in some formats, such as
SVG. Here, if using rgb-color, the opacity is optional as it defaults to 1,0.
There is also a convert-ly rule for changing rgb- to rgba-.