Add a makeDrawContextWithFallback that handles config fallback

include/gpu/GrContext.h

 /*
  * Copyright 2010 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef GrContext_DEFINED
 #define GrContext_DEFINED
 
@@ skipping 166 matching lines @@
      * @return sample count that should be perform well and have good enough
      *         rendering quality for the display. Alternatively returns 0 if
      *         MSAA is not supported or recommended to be used by default.
      */
     int getRecommendedSampleCount(GrPixelConfig config, SkScalar dpi) const;
 
     /**
      * Create both a GrRenderTarget and a matching GrDrawContext to wrap it.
      * We guarantee that "asTexture" will succeed for drawContexts created
      * via this entry point.
+     * This method will check the renderability of 'config' and fallback to

[bsalomon, 2016/09/01 19:47:05]

Should we specifically call out that the set of channels may be a superset of
the request but that the data type for the requested channels will be the same?
Main concern is that a caller won't think they might get an 8bit unorm channel
when they asked for a float16 (for example).

[robertphillips, 2016/09/01 20:52:54]

Done.

+     * an alternate config if necessary (e.g., A8 and 888 -> 8888)
      */
     sk_sp<GrDrawContext> makeDrawContext(SkBackingFit fit, 
                                          int width, int height,
                                          GrPixelConfig config,
                                          sk_sp<SkColorSpace> colorSpace,
                                          int sampleCnt = 0,
                                          GrSurfaceOrigin origin = kDefault_GrSurfaceOrigin,
                                          const SkSurfaceProps* surfaceProps = nullptr,
                                          SkBudgeted = SkBudgeted::kYes);
 
@@ skipping 286 matching lines @@
     SkAutoTUnref<const GrCaps>  fCaps;
     uint32_t                    fContextUniqueID;
 
     friend class GrContext;
     friend class SkImage;
 
     typedef SkRefCnt INHERITED;
 };
 
 #endif