Force input coverage to be only a byte in gpu shaders.

src/gpu/GrPaint.cpp

Index: src/gpu/GrPaint.cpp
diff --git a/src/gpu/GrPaint.cpp b/src/gpu/GrPaint.cpp
index ffde5f99e4116dbf920065d4c529ce481435da3d..94ea29195dbdc35a14bb6d2051fcaf0183c35a5c 100644
--- a/src/gpu/GrPaint.cpp
+++ b/src/gpu/GrPaint.cpp
@@ -55,7 +55,7 @@ bool GrPaint::getOpaqueAndKnownColor(GrColor* solidColor,
     GrProcessor::InvariantOutput inout;
     inout.fColor = GrColorPackRGBA(fCoverage, fCoverage, fCoverage, fCoverage);
     inout.fValidFlags = kRGBA_GrColorComponentFlags;
-    inout.fIsSingleComponent = false;
+    inout.fIsSingleComponent = true;
     int count = fCoverageStages.count();
     for (int i = 0; i < count; ++i) {
         fCoverageStages[i].getProcessor()->computeInvariantOutput(&inout);