Fix per-vertex coverage on Macs with aligned attrib bug

Review URL: http://codereview.appspot.com/5250067



git-svn-id: http://skia.googlecode.com/svn/trunk@2458 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/gpu/src/GrGpuGLShaders.cpp b/gpu/src/GrGpuGLShaders.cpp
index 3ca4346..9c0c9cc 100644
--- a/gpu/src/GrGpuGLShaders.cpp
+++ b/gpu/src/GrGpuGLShaders.cpp
@@ -802,9 +802,8 @@
     }
 
     if (newCoverageOffset > 0) {
-        // bind just alpha channel.
-        GrGLvoid* coverageOffset = (int8_t*)(vertexOffset + newCoverageOffset +
-                                             GrColor_INDEX_A);
+        // bind a single channel, they should all have the same value.
+        GrGLvoid* coverageOffset = (int8_t*)(vertexOffset + newCoverageOffset);
         int idx = GrGLProgram::CoverageAttributeIdx();
         if (oldCoverageOffset <= 0) {
             GL_CALL(EnableVertexAttribArray(idx));