Add SkPMColor4f support to SkSL

Convert GrConstColorProcessor to store SkPMColor4f

Bug: skia:
Change-Id: I6c505856653a02e576ae11fca59dc307545437f7
Reviewed-on: https://skia-review.googlesource.com/c/159152
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
diff --git a/src/gpu/GrFragmentProcessor.h b/src/gpu/GrFragmentProcessor.h
index f82146d..19ce61b 100644
--- a/src/gpu/GrFragmentProcessor.h
+++ b/src/gpu/GrFragmentProcessor.h
@@ -341,7 +341,7 @@
 private:
     virtual SkPMColor4f constantOutputForConstantInput(const SkPMColor4f& /* inputColor */) const {
         SK_ABORT("Subclass must override this if advertising this optimization.");
-        return { 0, 0, 0, 0 };
+        return SK_PMColor4fTRANSPARENT;
     }
 
     /** Returns a new instance of the appropriate *GL* implementation class