Add a makeDrawContextWithFallback that handles config fallback

src/gpu/GrClipStackClip.cpp

 /*
  * Copyright 2016 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "GrClipStackClip.h"
 
 #include "GrAppliedClip.h"
@@ skipping 395 matching lines @@
 
 sk_sp<GrTexture> GrClipStackClip::CreateAlphaClipMask(GrContext* context,
                                                       const GrReducedClip& reducedClip) {
     GrResourceProvider* resourceProvider = context->resourceProvider();
     GrUniqueKey key;
     GetClipMaskKey(reducedClip.elementsGenID(), reducedClip.ibounds(), &key);
     if (GrTexture* texture = resourceProvider->findAndRefTextureByUniqueKey(key)) {
         return sk_sp<GrTexture>(texture);
     }
 
-    // There's no texture in the cache. Let's try to allocate it then.
-    GrPixelConfig config = kRGBA_8888_GrPixelConfig;
-    if (context->caps()->isConfigRenderable(kAlpha_8_GrPixelConfig, false)) {
-        config = kAlpha_8_GrPixelConfig;
-    }
-
     sk_sp<GrDrawContext> dc(context->makeDrawContext(SkBackingFit::kApprox,
                                                      reducedClip.width(),
                                                      reducedClip.height(),
-                                                     config, nullptr));
+                                                     kAlpha_8_GrPixelConfig, nullptr));
     if (!dc) {
         return nullptr;
     }
 
     if (!reducedClip.drawAlphaClipMask(dc.get())) {
         return nullptr;
     }
 
     sk_sp<GrTexture> texture(dc->asTexture());
     SkASSERT(texture);
@@ skipping 66 matching lines @@
     sk_sp<GrTexture> result(texProvider->createApproxTexture(desc));
     if (!result) {
         return nullptr;
     }
     result->resourcePriv().setUniqueKey(key);
 
     helper.toTexture(result.get());
 
     return result;
 }