Make XPFragmentBuilder only Builder with access to DstCopy.
Plus a bunch of renaming.

BUG=skia:

Review URL: https://codereview.chromium.org/1110033004
diff --git a/src/gpu/effects/GrOvalEffect.cpp b/src/gpu/effects/GrOvalEffect.cpp
index 69f6326..d387b67 100644
--- a/src/gpu/effects/GrOvalEffect.cpp
+++ b/src/gpu/effects/GrOvalEffect.cpp
@@ -135,7 +135,7 @@
                                          "circle",
                                          &circleName);
 
-    GrGLFPFragmentBuilder* fsBuilder = builder->getFragmentShaderBuilder();
+    GrGLFragmentBuilder* fsBuilder = builder->getFragmentShaderBuilder();
     const char* fragmentPos = fsBuilder->fragmentPosition();
 
     SkASSERT(kHairlineAA_GrProcessorEdgeType != ce.getEdgeType());
@@ -317,7 +317,7 @@
                                          "ellipse",
                                          &ellipseName);
 
-    GrGLFPFragmentBuilder* fsBuilder = builder->getFragmentShaderBuilder();
+    GrGLFragmentBuilder* fsBuilder = builder->getFragmentShaderBuilder();
     const char* fragmentPos = fsBuilder->fragmentPosition();
 
     // d is the offset to the ellipse center