add constant coverage

[bsalomon, 2012-01-17 17:01:25]

Today we have two ways of specifying colors to a GrDrawTarget draw:
1. Set constant color on GrDrawState
2. per-vertex-colors specified in vertex layout

where 2) overrides 1). For coverage we support per-vertex values but have to way
to specify a constant coverage. We'll want constant coverage when rendering
hairlines that are thinner than 1 pixel (today we tweak the alpha value which is
incompatible with some xfermodes).

This adds a constant coverage on GrDrawState.

[TomH, 2012-01-17 19:36:05]

Minor bug? Fix that and LGTM.

http://codereview.appspot.com/5543052/diff/1010/src/gpu/GrDrawState.h
File src/gpu/GrDrawState.h (right):

http://codereview.appspot.com/5543052/diff/1010/src/gpu/GrDrawState.h#newcode134
src/gpu/GrDrawState.h:134: * to 1 (0xff). This value is ignored when there is
per-vertex coverage.
BUG: Default value mentioned in comments but not actually provided in function
declaration?

http://codereview.appspot.com/5543052/diff/1010/src/gpu/GrDrawState.h#newcode143
src/gpu/GrDrawState.h:143: */
If uint8_t version has a default value, should this one as well? (I don't
*think* so, but want to make sure, given apparent accidental omission above)