store SkAlphaType inside SkBitmap, on road to support unpremul

src/gpu/SkGpuDevice.cpp

Index: src/gpu/SkGpuDevice.cpp
diff --git a/src/gpu/SkGpuDevice.cpp b/src/gpu/SkGpuDevice.cpp
index 8cf2e4b664320dea8444be7a3d6da899ae21ee17..663f984054b8e75e513bd9740eae84c491a1e6f4 100644
--- a/src/gpu/SkGpuDevice.cpp
+++ b/src/gpu/SkGpuDevice.cpp
@@ -138,13 +138,12 @@ static SkBitmap::Config grConfig2skConfig(GrPixelConfig config, bool* isOpaque)
 }
 
 static SkBitmap make_bitmap(GrContext* context, GrRenderTarget* renderTarget) {
-    GrPixelConfig config = renderTarget->config();
-
     bool isOpaque;
+    SkBitmap::Config config = grConfig2skConfig(renderTarget->config(), &isOpaque);
+
     SkBitmap bitmap;
-    bitmap.setConfig(grConfig2skConfig(config, &isOpaque),
-                     renderTarget->width(), renderTarget->height());
-    bitmap.setIsOpaque(isOpaque);
+    bitmap.setConfig(config, renderTarget->width(), renderTarget->height(), 0,
+                     isOpaque ? kOpaque_SkAlphaType : kPremul_SkAlphaType);
     return bitmap;
 }