Add GrPaint::*StageEnabled() and GrDrawState::stageEnabled() functions.
These wrap the question of "is this stage of the shader enabled?" so that
we can change the semantics - previously iff there was a texture, now
if there is a texture OR a GrCustomStage, soon (post-cl 6306097) iff there
is a GrCustomStage, which at that point will hold whatever texture is
necessary.

http://codereview.appspot.com/6306104/



git-svn-id: http://skia.googlecode.com/svn/trunk@4325 2bbb7eff-a529-9590-31e7-b0007b416f81
7 files changed