Introduce SkRGBA4f, templated on SkAlphaType

Most functionality is shared, but this lets us get type safety.
SkColor4f = SkRGBA4f<kUnpremul>
SkPMColor4f = SkRGBA4f<kPremul>

Change-Id: I27408565dc92e722f42a185cecbf7af15d1dde3f
Reviewed-on: https://skia-review.googlesource.com/156243
Commit-Queue: Brian Osman <brianosman@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
Auto-Submit: Mike Klein <mtklein@google.com>
diff --git a/src/gpu/effects/GrXfermodeFragmentProcessor.cpp b/src/gpu/effects/GrXfermodeFragmentProcessor.cpp
index b5e3d13..9014114 100644
--- a/src/gpu/effects/GrXfermodeFragmentProcessor.cpp
+++ b/src/gpu/effects/GrXfermodeFragmentProcessor.cpp
@@ -151,10 +151,10 @@
         input = input.opaque();
         GrColor4f srcColor = ConstantOutputForConstantInput(this->childProcessor(0), input);
         GrColor4f dstColor = ConstantOutputForConstantInput(this->childProcessor(1), input);
-        SkColor4f src = srcColor.toSkColor4f();
-        SkColor4f dst = dstColor.toSkColor4f();
-        SkColor4f res = SkBlendMode_Apply(fMode, src, dst);
-        return GrColor4f::FromSkColor4f(res).mulByScalar(alpha);
+        SkPMColor4f src = srcColor.asRGBA4f<kPremul_SkAlphaType>();
+        SkPMColor4f dst = dstColor.asRGBA4f<kPremul_SkAlphaType>();
+        SkPMColor4f res = SkBlendMode_Apply(fMode, src, dst);
+        return GrColor4f::FromRGBA4f(res).mulByScalar(alpha);
     }
 
     GrGLSLFragmentProcessor* onCreateGLSLInstance() const override;
@@ -406,16 +406,16 @@
     GrColor4f constantOutputForConstantInput(GrColor4f inputColor) const override {
         GrColor4f childColor =
                 ConstantOutputForConstantInput(this->childProcessor(0), GrColor4f::OpaqueWhite());
-        SkColor4f src, dst;
+        SkPMColor4f src, dst;
         if (kSrc_Child == fChild) {
-            src = childColor.toSkColor4f();
-            dst = inputColor.toSkColor4f();
+            src = childColor.asRGBA4f<kPremul_SkAlphaType>();
+            dst = inputColor.asRGBA4f<kPremul_SkAlphaType>();
         } else {
-            src = inputColor.toSkColor4f();
-            dst = childColor.toSkColor4f();
+            src = inputColor.asRGBA4f<kPremul_SkAlphaType>();
+            dst = childColor.asRGBA4f<kPremul_SkAlphaType>();
         }
-        SkColor4f res = SkBlendMode_Apply(fMode, src, dst);
-        return GrColor4f::FromSkColor4f(res);
+        SkPMColor4f res = SkBlendMode_Apply(fMode, src, dst);
+        return GrColor4f::FromRGBA4f(res);
     }
 
 private: