Add a makeDrawContextWithFallback that handles config fallback

src/gpu/GrBlurUtils.cpp

 /*
  * Copyright 2015 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "GrBlurUtils.h"
 #include "GrDrawContext.h"
 #include "GrCaps.h"
@@ skipping 86 matching lines @@
                                         const SkIRect& maskRect,
                                         const SkPath& devPath,
                                         SkStrokeRec::InitStyle fillOrHairline,
                                         bool doAA,
                                         int sampleCnt) {
     if (!doAA) {
         // Don't need MSAA if mask isn't AA
         sampleCnt = 0;
     }
 
-    // We actually only need A8, but it often isn't supported as a
-    // render target so default to RGBA_8888
-    GrPixelConfig config = kRGBA_8888_GrPixelConfig;
-    if (context->caps()->isConfigRenderable(kAlpha_8_GrPixelConfig, sampleCnt > 0)) {
-        config = kAlpha_8_GrPixelConfig;
-    }
-
     sk_sp<GrDrawContext> drawContext(context->makeDrawContext(SkBackingFit::kApprox,
                                                               maskRect.width(), 
                                                               maskRect.height(),
-                                                              config,
+                                                              kAlpha_8_GrPixelConfig,
                                                               nullptr,
                                                               sampleCnt));
     if (!drawContext) {
         return nullptr;
     }
 
     drawContext->clear(nullptr, 0x0, true);
 
     GrPaint tempPaint;
     tempPaint.setAntiAlias(doAA);
@@ skipping 166 matching lines @@
     }
 
     if (paint.getMaskFilter()) {
         draw_path_with_mask_filter(context, drawContext, clip, &grPaint, viewMatrix,
                                    paint.getMaskFilter(), style,
                                    path, pathIsMutable);
     } else {
         drawContext->drawPath(clip, grPaint, viewMatrix, *path, style);
     }
 }