extend compose-colorfilter to 4f

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1664663003

Review URL: https://codereview.chromium.org/1664663003
diff --git a/include/core/SkColor.h b/include/core/SkColor.h
index 461a538..90453f5 100644
--- a/include/core/SkColor.h
+++ b/include/core/SkColor.h
@@ -160,6 +160,8 @@
 
 ///////////////////////////////////////////////////////////////////////////////////////////////////
 
+struct SkColor4f;
+
 /*
  *  The float values are 0...1 premultiplied
  */
@@ -174,6 +176,8 @@
 
     float a() const { return fVec[A]; }
 
+    SkColor4f unpremul() const;
+
     static SkPM4f FromPMColor(SkPMColor);
 
 #ifdef SK_DEBUG