Update GrFillBoundingBoxShader to use attribs

Uses an instanced attrib instead of a uniform to access the bounding
box. This will allow for batching and fewer uniform updates.

Bug: skia:10419
Change-Id: I522b4a29835564f89294a02ee4edc7a866812164
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/411976
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
Commit-Queue: Chris Dalton <csmartdalton@google.com>
diff --git a/src/gpu/tessellate/GrPathStencilFillOp.h b/src/gpu/tessellate/GrPathStencilFillOp.h
index cfecd47..a3f2d7e 100644
--- a/src/gpu/tessellate/GrPathStencilFillOp.h
+++ b/src/gpu/tessellate/GrPathStencilFillOp.h
@@ -68,6 +68,9 @@
     int fFanBaseVertex = 0;
     int fFanVertexCount = 0;
 
+    sk_sp<const GrBuffer> fBBoxBuffer;
+    int fBBoxBaseInstance = 0;
+
     friend class GrOp;  // For ctor.
 };