Make SkGpuDevice know its alpha type

src/image/SkImage_Gpu.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 "SkImage_Gpu.h"
 #include "GrContext.h"
 #include "GrDrawContext.h"
@@ skipping 43 matching lines @@
         return false;
     }
     if (!fTexture->readPixels(0, 0, dst->width(), dst->height(), kSkia8888_GrPixelConfig,
                               dst->getPixels(), dst->rowBytes())) {
         return false;
     }
     return true;
 }
 
 bool SkImage_Gpu::isOpaque() const {
-    return GrPixelConfigIsOpaque(fTexture->config());
+    return GrPixelConfigIsOpaque(fTexture->config()) || fAlphaType == kOpaque_SkAlphaType;
 }
 
 static void apply_premul(const SkImageInfo& info, void* pixels, size_t rowBytes) {
     switch (info.colorType()) {
         case kRGBA_8888_SkColorType:
         case kBGRA_8888_SkColorType:
             break;
         default:
             return; // nothing to do
     }
@@ skipping 174 matching lines @@
                                                      colorSpace))->unref();
 
     const SkRect rect = SkRect::MakeWH(SkIntToScalar(dstDesc.fWidth),
                                        SkIntToScalar(dstDesc.fHeight));
     GrDrawContext* drawContext = ctx->drawContext();
     drawContext->drawRect(dst->asRenderTarget(), GrClip::WideOpen(), paint, SkMatrix::I(), rect);
     ctx->flushSurfaceWrites(dst);
     return SkNEW_ARGS(SkImage_Gpu, (dstDesc.fWidth, dstDesc.fHeight, kOpaque_SkAlphaType, dst, 0,
                                     budgeted));
 }