Introduce GrBatch subclasses GrDrawBatch and GrVertexBatch to prepare for non-drawing batches

tests/GrPorterDuffTest.cpp

Index: tests/GrPorterDuffTest.cpp
diff --git a/tests/GrPorterDuffTest.cpp b/tests/GrPorterDuffTest.cpp
index 0cd1db0131d84318a9de163d121d40f7ead476ee..c276e592f5c14f5ba69761eac10d587a8a885059 100644
--- a/tests/GrPorterDuffTest.cpp
+++ b/tests/GrPorterDuffTest.cpp
@@ -898,7 +898,7 @@ static void test_color_opaque_no_coverage(skiatest::Reporter* reporter, const Gr
 }
 
 static void test_lcd_coverage(skiatest::Reporter* reporter, const GrCaps& caps) {
-    class : public GrBatch {
+    class : public GrVertexBatch {
         void getInvariantOutputColor(GrInitInvariantOutput* out) const override {
             out->setKnownFourComponents(GrColorPackRGBA(123, 45, 67, 221));
         }