Add a makeDrawContextWithFallback that handles config fallback

src/gpu/GrContext.cpp

Index: src/gpu/GrContext.cpp
diff --git a/src/gpu/GrContext.cpp b/src/gpu/GrContext.cpp
index 015776099038649fd1134e0974329ec17028a2c1..a25bcf1dc5acf599284ed3a8821afa73c9350c2c 100644
--- a/src/gpu/GrContext.cpp
+++ b/src/gpu/GrContext.cpp
@@ -689,6 +689,29 @@ sk_sp<GrDrawContext> GrContext::makeDrawContext(SkBackingFit fit,
                                                 GrSurfaceOrigin origin,
                                                 const SkSurfaceProps* surfaceProps,
                                                 SkBudgeted budgeted) {
+    // If the config isn't renderable try converting to either A8 or an 32 bit config. Otherwise,

[bsalomon, 2016/09/01 19:47:05]

I think this should be more narrowly defined as always preserving the
representation for the channels in the requested config (e.g. not allowing 8bit
channels to substitute for 16 bit or 4 bit). We also need to be careful about
losing sRGBness.

[robertphillips, 2016/09/01 20:52:54]

Done.

+    // fail.
+    if (!this->caps()->isConfigRenderable(config, sampleCnt > 0)) {
+        if (GrPixelConfigIsAlphaOnly(config)) {
+            if (this->caps()->isConfigRenderable(kAlpha_8_GrPixelConfig, sampleCnt > 0)) {
+                config = kAlpha_8_GrPixelConfig;
+            } else if (this->caps()->isConfigRenderable(kSkia8888_GrPixelConfig, sampleCnt > 0)) {

[bsalomon, 2016/09/01 19:47:05]

Let's not use kSkia8888. I'd like to remove it. kRGBA_8888 is renderable more
often than kSkia8888.

[robertphillips, 2016/09/01 20:52:54]

Done.

+                config = kSkia8888_GrPixelConfig;
+            } else {
+                return nullptr;
+            }
+        } else if (kRGB_GrColorComponentFlags ==
+                (kRGB_GrColorComponentFlags & GrPixelConfigComponentMask(config))) {
+            if (this->caps()->isConfigRenderable(kSkia8888_GrPixelConfig, sampleCnt > 0)) {
+                config = kSkia8888_GrPixelConfig;
+            } else {
+                return nullptr;
+            }
+        } else {
+            return nullptr;
+        }
+    }
+
     GrSurfaceDesc desc;
     desc.fFlags = kRenderTarget_GrSurfaceFlag;
     desc.fOrigin = origin;