Add more specialized fragment builders

Adds specialized fragment builders for primitive and fragment
processors. Removes fragment-specific functionality from the Xfer
fragment builder.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1709153002

Review URL: https://codereview.chromium.org/1709153002
diff --git a/src/effects/SkMorphologyImageFilter.cpp b/src/effects/SkMorphologyImageFilter.cpp
index 2a2b76e..61b0c33 100644
--- a/src/effects/SkMorphologyImageFilter.cpp
+++ b/src/effects/SkMorphologyImageFilter.cpp
@@ -281,7 +281,7 @@
                                            "Range");
     const char* range = uniformHandler->getUniformCStr(fRangeUni);
 
-    GrGLSLFragmentBuilder* fragBuilder = args.fFragBuilder;
+    GrGLSLFPFragmentBuilder* fragBuilder = args.fFragBuilder;
     SkString coords2D = fragBuilder->ensureFSCoords2D(args.fCoords, 0);
     const char* func;
     switch (me.type()) {