Fix alpha textures in NV ES3 contexts on Windows

tests/ImageNewShaderTest.cpp

 /*
  * Copyright 2014 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkTypes.h"
 #if SK_SUPPORT_GPU
 #include "GrContextFactory.h"
@@ skipping 74 matching lines @@
 void rasterToGpu(skiatest::Reporter* reporter, GrContext* context) {
     SkImageInfo info = SkImageInfo::MakeN32Premul(5, 5);
 
     SkAutoTUnref<SkSurface> srcSurface(SkSurface::NewRaster(info));
     SkAutoTUnref<SkSurface> dstSurface(SkSurface::NewRenderTarget(context, info));
 
     runShaderTest(reporter, srcSurface.get(), dstSurface.get(), info);
 }
 
 DEF_GPUTEST(ImageNewShader_GPU, reporter, factory) {
-    GrContext* context = factory->get(GrContextFactory::kNative_GLContextType);
+    for (int i= 0; i < GrContextFactory::kGLContextTypeCnt; ++i) {
+        GrContextFactory::GLContextType glCtxType = (GrContextFactory::GLContextType) i;
 
-    // GPU -> GPU
-    gpuToGpu(reporter, context);
+        if (!GrContextFactory::IsRenderingGLContext(glCtxType)) {
+            continue;
+        }
 
-    // GPU -> RASTER
-    gpuToRaster(reporter, context);
+        GrContext* context = factory->get(glCtxType);
+
+        if (NULL == context) {
+            continue;
+        }
+
+        // GPU -> GPU
+        gpuToGpu(reporter, context);
+
+        // GPU -> RASTER
+        gpuToRaster(reporter, context);
 
 
-    // RASTER -> GPU
-    rasterToGpu(reporter, context);
+        // RASTER -> GPU
+        rasterToGpu(reporter, context);
+    }
 }
 
 #endif