Introduce GrColorSpaceXform, for gamut conversion on textures

src/effects/GrAlphaThresholdFragmentProcessor.cpp

Index: src/effects/GrAlphaThresholdFragmentProcessor.cpp
diff --git a/src/effects/GrAlphaThresholdFragmentProcessor.cpp b/src/effects/GrAlphaThresholdFragmentProcessor.cpp
index 03a451519151c3593a4b9e58d4d9631c020716a7..e6a4013a32b3cff420dd2861b9449bac80a88f21 100644
--- a/src/effects/GrAlphaThresholdFragmentProcessor.cpp
+++ b/src/effects/GrAlphaThresholdFragmentProcessor.cpp
@@ -44,12 +44,12 @@ GrAlphaThresholdFragmentProcessor::GrAlphaThresholdFragmentProcessor(GrTexture*
     , fImageCoordTransform(kLocal_GrCoordSet,
                            GrCoordTransform::MakeDivByTextureWHMatrix(texture), texture,
                            GrTextureParams::kNone_FilterMode)
-    , fImageTextureAccess(texture)
+    , fImageTextureAccess(texture, nullptr)
     , fMaskCoordTransform(kLocal_GrCoordSet,
                           make_div_and_translate_matrix(maskTexture, -bounds.x(), -bounds.y()),
                           maskTexture,
                           GrTextureParams::kNone_FilterMode)
-    , fMaskTextureAccess(maskTexture) {
+    , fMaskTextureAccess(maskTexture, nullptr) {
     this->initClassID<GrAlphaThresholdFragmentProcessor>();
     this->addCoordTransform(&fImageCoordTransform);
     this->addTextureAccess(&fImageTextureAccess);