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

src/gpu/GrAARectRenderer.cpp

Index: src/gpu/GrAARectRenderer.cpp
diff --git a/src/gpu/GrAARectRenderer.cpp b/src/gpu/GrAARectRenderer.cpp
index d4aad406c61ad56b0a1a6c42de8e5baa39526732..41eb77cf821011750fee0f95a5134ec4d18f4c88 100644
--- a/src/gpu/GrAARectRenderer.cpp
+++ b/src/gpu/GrAARectRenderer.cpp
@@ -265,7 +265,7 @@ namespace {
 extern const GrVertexAttrib gAARectAttribs[] = {
     {kVec2f_GrVertexAttribType,  0,                                 kPosition_GrVertexAttribBinding},
     {kVec4ub_GrVertexAttribType, sizeof(SkPoint),                   kColor_GrVertexAttribBinding},
-    {kVec4ub_GrVertexAttribType, sizeof(SkPoint) + sizeof(SkColor), kCoverage_GrVertexAttribBinding},
+    {kUByte_GrVertexAttribType, sizeof(SkPoint) + sizeof(SkColor), kCoverage_GrVertexAttribBinding},
 };
 
 // Should the coverage be multiplied into the color attrib or use a separate attrib.