starter procs for blending with pm4f

src/core/SkColor.cpp

Index: src/core/SkColor.cpp
diff --git a/src/core/SkColor.cpp b/src/core/SkColor.cpp
index 1794cd7d909c68a23f102d25cfe9e4c73ed0ed2f..9deac7a197689d9bd2f5a41b8c8bb9e429a49b48 100644
--- a/src/core/SkColor.cpp
+++ b/src/core/SkColor.cpp
@@ -141,3 +141,8 @@ SkPM4f SkColor4f::premul() const {
     dst.store(pm4.fVec);
     return pm4;
 }
+
+bool SkPM4f::isUnit() const {

[mtklein, 2016/01/29 15:23:00]

If this is only used inside asserts, we may want to power this down to

#ifdef SK_DEBUG
    void assertIsUnit() const;
#else
   void assertIsUnit() const {}
#endif

allTrue() is the best way to do this sort of thing, but it's not exactly fast
(especially on ARM).  Best to not use in hot code.

[reed1, 2016/01/29 22:35:27]

Gotcha. No idea if we'll need it for real, so for now I'll just make it a
debug-assert.

+    auto c4 = Sk4f::Load(fVec);
+    return (c4 >= Sk4f(0)).allTrue() && (c4 <= Sk4f(1)).allTrue();
+}