Add a makeDrawContextWithFallback that handles config fallback

include/gpu/GrContext.h

Index: include/gpu/GrContext.h
diff --git a/include/gpu/GrContext.h b/include/gpu/GrContext.h
index 5785553dfdeae753b36233d1c81b2e3bfa663cea..545c3e135b930eabde4c39ed783f4563c0558f3c 100644
--- a/include/gpu/GrContext.h
+++ b/include/gpu/GrContext.h
@@ -184,6 +184,8 @@ public:
      * 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,