Decouple contrast boost from fake gamma.

src/core/SkScalerContext.h

Index: src/core/SkScalerContext.h
diff --git a/src/core/SkScalerContext.h b/src/core/SkScalerContext.h
index f2df93cef31ad611c79fd8bf3f9c003778753829..331d1df21db6df72cc724b98919bbab0372a60ea 100644
--- a/src/core/SkScalerContext.h
+++ b/src/core/SkScalerContext.h
@@ -71,11 +71,13 @@ struct SkScalerContextRec {
      *  Causes the luminance color and contrast to be ignored, and the
      *  paint and device gamma to be effectively 1.0.
      */
-    void ignorePreBlend() {
+    void ignorePreBlend(bool keepContrast) {
         setLuminanceColor(SK_ColorTRANSPARENT);
         setPaintGamma(SK_Scalar1);
         setDeviceGamma(SK_Scalar1);
-        setContrast(0);
+        if (!keepContrast) {
+            setContrast(0);
+        }
     }
 
     uint8_t     fMaskFormat;