add new copyTo version to SkBitmap, which takes SkColorType

src/image/SkImage_Raster.cpp

Index: src/image/SkImage_Raster.cpp
diff --git a/src/image/SkImage_Raster.cpp b/src/image/SkImage_Raster.cpp
index 504943e8943b728d150813fb1039eddb74ffb886..abf6dc8af08a761914aa59a29bed9cb0a7705fa0 100644
--- a/src/image/SkImage_Raster.cpp
+++ b/src/image/SkImage_Raster.cpp
@@ -127,7 +127,7 @@ bool SkImage_Raster::onReadPixels(SkBitmap* dst, const SkIRect& subset) const {
         if (!fBitmap.extractSubset(&src, subset)) {
             return false;
         }
-        return src.copyTo(dst, src.config());
+        return src.copyTo(dst, src.colorType());
     }
 }