Modify GrBWFillRectBatch to use GrQuad

BUG=skia:

Committed: https://skia.googlesource.com/skia/+/c611b5afbef6df6c02e949fdf092aab2bb7e0e2e

Review URL: https://codereview.chromium.org/1311793002
diff --git a/src/gpu/GrQuad.h b/src/gpu/GrQuad.h
index fc169ea..9a8d1f3 100644
--- a/src/gpu/GrQuad.h
+++ b/src/gpu/GrQuad.h
@@ -16,6 +16,8 @@
  */
 class GrQuad {
 public:
+    GrQuad() {}
+
     GrQuad(const GrQuad& that) {
         *this = that;
     }
@@ -50,6 +52,11 @@
         return fPoints;
     }
 
+    const SkPoint& point(int i) const {
+        SkASSERT(i < kNumPoints);
+        return fPoints[i];
+    }
+
 private:
     static const int kNumPoints = 4;
     SkPoint fPoints[kNumPoints];