Fix some GPU image filter code to preserve precision and color space

src/gpu/SkGrPriv.h

Index: src/gpu/SkGrPriv.h
diff --git a/src/gpu/SkGrPriv.h b/src/gpu/SkGrPriv.h
index 26dc4aaa86af77093ef8370afe361199252514f7..c1069d1a207d19ea934e8622442b54e63c8edf27 100644
--- a/src/gpu/SkGrPriv.h
+++ b/src/gpu/SkGrPriv.h
@@ -105,6 +105,11 @@ GrSurfaceDesc GrImageInfoToSurfaceDesc(const SkImageInfo&, const GrCaps&);
 
 bool GrPixelConfigToColorType(GrPixelConfig, SkColorType*);
 
+/** When image filter code needs to construct a draw context to do intermediate rendering, we need
+    a renderable pixel config. The source (SkSpecialImage) may not be in a renderable format, but
+    we want to preserve the color space of that source. This picks an appropriate format to use. */

[robertphillips, 2016/09/21 13:44:10]

Should this take a const SkColorSpace* ?

+GrPixelConfig GrRenderableConfigForColorSpace(SkColorSpace*);
+
 /**
  *  If the compressed data in the SkData is supported (as a texture format, this returns
  *  the pixel-config that should be used, and sets outStartOfDataToUpload to the ptr into