Add per-vertex coverage field, use in AA rects when alpha tweak is not valid, add relevant sample/gm

Review URL: http://codereview.appspot.com/5181044/



git-svn-id: http://skia.googlecode.com/svn/trunk@2440 2bbb7eff-a529-9590-31e7-b0007b416f81
diff --git a/gpu/src/GrGLProgram.h b/gpu/src/GrGLProgram.h
index a79f877..197da5d 100644
--- a/gpu/src/GrGLProgram.h
+++ b/gpu/src/GrGLProgram.h
@@ -66,7 +66,10 @@
     static int PositionAttributeIdx() { return 0; }
     static int TexCoordAttributeIdx(int tcIdx) { return 1 + tcIdx; }
     static int ColorAttributeIdx() { return 1 + GrDrawTarget::kMaxTexCoords; }
-    static int EdgeAttributeIdx() { return 2 + GrDrawTarget::kMaxTexCoords; }
+    static int CoverageAttributeIdx() {
+        return 2 + GrDrawTarget::kMaxTexCoords;
+    }
+    static int EdgeAttributeIdx() { return 3 + GrDrawTarget::kMaxTexCoords; }
 
     static int ViewMatrixAttributeIdx() {
         return 2 + GrDrawTarget::kMaxTexCoords;