Add a makeDrawContextWithFallback that handles config fallback

src/effects/SkAlphaThresholdFilter.cpp

 /*
  * Copyright 2013 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkAlphaThresholdFilter.h"
 
 #include "SkBitmap.h"
@@ skipping 78 matching lines @@
     : INHERITED(&input, 1, cropRect)
     , fRegion(region)
     , fInnerThreshold(innerThreshold)
     , fOuterThreshold(outerThreshold) {
 }
 
 #if SK_SUPPORT_GPU
 sk_sp<GrTexture> SkAlphaThresholdFilterImpl::createMaskTexture(GrContext* context,
                                                                const SkMatrix& inMatrix,
                                                                const SkIRect& bounds) const {
-    GrPixelConfig config;
-    if (context->caps()->isConfigRenderable(kAlpha_8_GrPixelConfig, false)) {
-        config = kAlpha_8_GrPixelConfig;
-    } else {
-        config = kRGBA_8888_GrPixelConfig;
-    }
 
     sk_sp<GrDrawContext> drawContext(context->makeDrawContext(SkBackingFit::kApprox,
                                                               bounds.width(), bounds.height(),
-                                                              config, nullptr));
+                                                              kAlpha_8_GrPixelConfig, nullptr));
     if (!drawContext) {
         return nullptr;
     }
 
     GrPaint grPaint;
     grPaint.setPorterDuffXPFactory(SkXfermode::kSrc_Mode);
     SkRegion::Iterator iter(fRegion);
     drawContext->clear(nullptr, 0x0, true);
 
     GrFixedClip clip(SkIRect::MakeWH(bounds.width(), bounds.height()));
@@ skipping 141 matching lines @@
                                           dst);
 }
 
 #ifndef SK_IGNORE_TO_STRING
 void SkAlphaThresholdFilterImpl::toString(SkString* str) const {
     str->appendf("SkAlphaThresholdImageFilter: (");
     str->appendf("inner: %f outer: %f", fInnerThreshold, fOuterThreshold);
     str->append(")");
 }
 #endif