Add a makeDrawContextWithFallback that handles config fallback

src/gpu/GrTextureToYUVPlanes.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 "GrTextureToYUVPlanes.h"
 #include "effects/GrSimpleTextureEffect.h"
 #include "effects/GrYUVEffect.h"
@@ skipping 40 matching lines @@
                           const size_t rowBytes[3], SkYUVColorSpace colorSpace) {
     if (GrContext* context = texture->getContext()) {
         // Depending on the relative sizes of the y, u, and v planes we may do 1 to 3 draws/
         // readbacks.
         sk_sp<GrDrawContext> yuvDrawContext;
         sk_sp<GrDrawContext> yDrawContext;
         sk_sp<GrDrawContext> uvDrawContext;
         sk_sp<GrDrawContext> uDrawContext;
         sk_sp<GrDrawContext> vDrawContext;
 
-        GrPixelConfig singleChannelPixelConfig;
-        if (context->caps()->isConfigRenderable(kAlpha_8_GrPixelConfig, false)) {
-            singleChannelPixelConfig = kAlpha_8_GrPixelConfig;
-        } else {
-            singleChannelPixelConfig = kRGBA_8888_GrPixelConfig;
-        }
-
         // We issue draw(s) to convert from RGBA to Y, U, and V. All three planes may have different
         // sizes however we optimize for two other cases - all planes are the same (1 draw to YUV),
         // and U and V are the same but Y differs (2 draws, one for Y, one for UV).
         if (sizes[0] == sizes[1] && sizes[1] == sizes[2]) {
             yuvDrawContext = context->makeDrawContext(SkBackingFit::kApprox,
                                                       sizes[0].fWidth, sizes[0].fHeight,
                                                       kRGBA_8888_GrPixelConfig, nullptr);
             if (!yuvDrawContext) {
                 return false;
             }
         } else {
             yDrawContext = context->makeDrawContext(SkBackingFit::kApprox,
                                                     sizes[0].fWidth, sizes[0].fHeight,
-                                                    singleChannelPixelConfig, nullptr);
+                                                    kAlpha_8_GrPixelConfig, nullptr);
             if (!yDrawContext) {
                 return false;
             }
             if (sizes[1] == sizes[2]) {
                 // TODO: Add support for GL_RG when available.
                 uvDrawContext = context->makeDrawContext(SkBackingFit::kApprox,
                                                          sizes[1].fWidth, sizes[1].fHeight,
                                                          kRGBA_8888_GrPixelConfig, nullptr);
                 if (!uvDrawContext) {
                     return false;
                 }
             } else {
                 uDrawContext = context->makeDrawContext(SkBackingFit::kApprox,
                                                         sizes[1].fWidth, sizes[1].fHeight,
-                                                        singleChannelPixelConfig, nullptr);
+                                                        kAlpha_8_GrPixelConfig, nullptr);
                 vDrawContext = context->makeDrawContext(SkBackingFit::kApprox,
                                                         sizes[2].fWidth, sizes[2].fHeight,
-                                                        singleChannelPixelConfig, nullptr);
+                                                        kAlpha_8_GrPixelConfig, nullptr);
                 if (!uDrawContext || !vDrawContext) {
                     return false;
                 }
             }
         }
 
         // Do all the draws before any readback.
         if (yuvDrawContext) {
             if (!convert_texture(texture, yuvDrawContext.get(),
                                  sizes[0].fWidth, sizes[0].fHeight,
@@ skipping 111 matching lines @@
                 if (!tex->readPixels(0, 0, sizes[2].fWidth, sizes[2].fHeight,
                                      kAlpha_8_GrPixelConfig, planes[2], rowBytes[2])) {
                     return false;
                 }
                 return true;
             }
         }
     }
     return false;
 }