Add a makeDrawContextWithFallback that handles config fallback

src/gpu/GrTextureToYUVPlanes.cpp

Index: src/gpu/GrTextureToYUVPlanes.cpp
diff --git a/src/gpu/GrTextureToYUVPlanes.cpp b/src/gpu/GrTextureToYUVPlanes.cpp
index 7a8647a86756254391c297bd49f0221500bcf0cc..08eed8af6bac74f976d58e9a13f06f6b4070a405 100644
--- a/src/gpu/GrTextureToYUVPlanes.cpp
+++ b/src/gpu/GrTextureToYUVPlanes.cpp
@@ -58,13 +58,6 @@ bool GrTextureToYUVPlanes(GrTexture* texture, const SkISize sizes[3], void* cons
         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).
@@ -78,7 +71,7 @@ bool GrTextureToYUVPlanes(GrTexture* texture, const SkISize sizes[3], void* cons
         } else {
             yDrawContext = context->makeDrawContext(SkBackingFit::kApprox,
                                                     sizes[0].fWidth, sizes[0].fHeight,
-                                                    singleChannelPixelConfig, nullptr);
+                                                    kAlpha_8_GrPixelConfig, nullptr);
             if (!yDrawContext) {
                 return false;
             }
@@ -93,10 +86,10 @@ bool GrTextureToYUVPlanes(GrTexture* texture, const SkISize sizes[3], void* cons
             } 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;
                 }