restore cache-count for gradients to p.o.2. so we can perform fancier dithering

restore cache-count for gradients to p.o.2. so we can perform fancier dithering
in the future.
Committed: https://code.google.com/p/skia/source/detail?r=7506

[reed1, 2013-01-31 22:07:58]

Need this cleanup before I can land my 2x2 dither stuff. Does not appear to
change any gms

[robertphillips, 2013-02-01 00:26:28]

lgtm

[TomH, 2013-02-01 13:20:53]

When I(?) put this in it was because of interpolation issues at the endpoints,
as the ex-comment states. Watch closely when you roll to Chrome?

[reed1, 2013-02-01 13:44:46]

On 2013/02/01 13:20:53, TomH wrote:
> When I(?) put this in it was because of interpolation issues at the endpoints,
> as the ex-comment states. Watch closely when you roll to Chrome?

Agreed, that is why I explicitly perform the range-check in the vertical_lerp
code (the only place we seemed to use lerp).

I confirmed that (a) we have GMs that fail if I skip that (new) code, and (b) w/
the new code the GMs pass.