add libpng source to repo and make Unix build use it

Imported libpng1.5.2 and added gyp file to build it.

This will allow us to remove our dependency on a system-level installation of libpng for Unix developers.  Next step is using this for Mac too, thus fixing gm tool.

Modifications from the stock distribution tar file within third_party/libpng :
- Addition of README.skia file
- cp scripts/pnglibconf.h.prebuilt pnglibconf.h

All Skia-specific build glue is in libpng.gyp, which is not kept in this directory.

I would have included this library via SVN externals like we do with "gyp", but libpng source is kept in a git repository (not svn).

[bsalomon, 2011-06-15 13:40:04]

On 2011/06/15 13:34:09, epoger wrote:
LGTM.

Maybe there is some git-svn wrapper magic thing that can make this work with
externals?

Let's get it working with Windows, too! Although many of us like to develop on
Linux and Mac, Windows is probably our most important platform.

[epoger, 2011-06-15 13:48:15]

On 2011/06/15 13:40:04, bsalomon wrote:
> On 2011/06/15 13:34:09, epoger wrote:
> LGTM.
> 
> Maybe there is some git-svn wrapper magic thing that can make this work with
> externals?

I'll try to look into this in the long term, but for now I have a high-priority
task of getting gm working again on Mac...

> 
> Let's get it working with Windows, too! Although many of us like to develop on
> Linux and Mac, Windows is probably our most important platform.

Sounds good.

[epoger, 2011-06-15 13:48:44]

committed as r1591