Implement support for mixed sampled render targets

src/gpu/gl/GrGLGpu.h

Index: src/gpu/gl/GrGLGpu.h
diff --git a/src/gpu/gl/GrGLGpu.h b/src/gpu/gl/GrGLGpu.h
index f6a48d7589869af0714572051c75f64ed0a8973c..73a6c497ed7e44232bec05085c1c919688571c44 100644
--- a/src/gpu/gl/GrGLGpu.h
+++ b/src/gpu/gl/GrGLGpu.h
@@ -448,6 +448,7 @@ private:
     } fHWBlendState;
 
     TriState fMSAAEnabled;
+    TriState fCoverageModulationEnabled;

[Mark Kilgard, 2015/04/08 03:20:09]

is fCoverageModulationMode a better name for this?

technically NV_framebuffer_mixed_sample's isn't really an "enable" but rather a
mode.

While GL_RGBA is most typically used, GL_NONE (initial value), GL_ALPHA, and
GL_RGB are also supported.

I expect Skia's use of pre-multiplied alpha everywhere means GL_RGBA is the
interesting mode for Skia so if GL_NONE and GL_RGBA are the only options used,
that is kind of like an enable.

[vbuzinov, 2015/04/08 12:05:30]

Since we only use GL_RGBA at this point I thought it would be easier to reuse
skia's TriState enum for it.  So it is either modulation, no modulation or
unknown.  Changing this to unknown or some value wouldn't be hard, though.

Come to think of it, Brian earlier mentioned that "coverage modulation" might
sound ambiguous with regards to skia's own notion of coverage.  Should I rename
the var to fMixedSamplesCoverageModulationEnabled or do you guys think that
would be unnecessary -- coverage doesn't come up in GrGLGpu otherwise?

 
     GrStencilSettings           fHWStencilSettings;
     TriState                    fHWStencilTestEnabled;