Check that coverage FPs are compatible with alpha as coverage

Change-Id: Ic3b6a02248d571e82f6729827c199d358fb75114
Reviewed-on: https://skia-review.googlesource.com/8508
Commit-Queue: Brian Salomon <bsalomon@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
diff --git a/src/gpu/effects/GrConvexPolyEffect.cpp b/src/gpu/effects/GrConvexPolyEffect.cpp
index 0248beb..838dff8 100644
--- a/src/gpu/effects/GrConvexPolyEffect.cpp
+++ b/src/gpu/effects/GrConvexPolyEffect.cpp
@@ -253,8 +253,12 @@
             return GrConstColorProcessor::Make(GrColor4f::OpaqueWhite(),
                                                GrConstColorProcessor::kModulateRGBA_InputMode);
         }
+        // This could use kIgnore instead of kModulateRGBA but it would trigger a debug print
+        // about a coverage processor not being compatible with the alpha-as-coverage optimization.
+        // We don't really care about this unlikely case so we just use kModulateRGBA to suppress
+        // the print.
         return GrConstColorProcessor::Make(GrColor4f::TransparentBlack(),
-                                           GrConstColorProcessor::kIgnore_InputMode);
+                                           GrConstColorProcessor::kModulateRGBA_InputMode);
     }
 
     SkVector t;