Add a makeDrawContextWithFallback that handles config fallback

src/effects/SkAlphaThresholdFilter.cpp

Index: src/effects/SkAlphaThresholdFilter.cpp
diff --git a/src/effects/SkAlphaThresholdFilter.cpp b/src/effects/SkAlphaThresholdFilter.cpp
index 5170a3d09880f8616afdb858b12dedcf27391090..7a41cc1d629c22f91a8cde9305aa2f64878a5630 100644
--- a/src/effects/SkAlphaThresholdFilter.cpp
+++ b/src/effects/SkAlphaThresholdFilter.cpp
@@ -96,16 +96,10 @@ SkAlphaThresholdFilterImpl::SkAlphaThresholdFilterImpl(const SkRegion& region,
 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;
     }