add isAlphaOnly() query

src/image/SkImage.cpp

 /*
  * Copyright 2012 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkBitmap.h"
 #include "SkBitmapCache.h"
 #include "SkCanvas.h"
@@ skipping 353 matching lines @@
     offset->fX += outSubset->x();
     offset->fY += outSubset->y();
     // This isn't really a "tight" subset, but includes any texture padding.
     return result->makeTightSubset(fullSize);
 }
 
 bool SkImage::isLazyGenerated() const {
     return as_IB(this)->onIsLazyGenerated();
 }
 
+bool SkImage::isAlphaOnly() const {
+    return as_IB(this)->onImageInfo().colorType() == kAlpha_8_SkColorType;
+}
+
 //////////////////////////////////////////////////////////////////////////////////////
 
 #if !SK_SUPPORT_GPU
 
 sk_sp<SkImage> SkImage::MakeTextureFromPixmap(GrContext*, const SkPixmap&, SkBudgeted budgeted) {
     return nullptr;
 }
 
 sk_sp<SkImage> MakeTextureFromMipMap(GrContext*, const SkImageInfo&, const GrMipLevel* texels,
                                      int mipLevelCount, SkBudgeted, SkSourceGammaTreatment) {
@@ skipping 124 matching lines @@
     SkASSERT(image);
     SkASSERT(ctx);
     as_IB(image)->onPinAsTexture(ctx);
 }
 
 void SkImage_unpinAsTexture(const SkImage* image, GrContext* ctx) {
     SkASSERT(image);
     SkASSERT(ctx);
     as_IB(image)->onUnpinAsTexture(ctx);
 }