Make A8 readback work in more cases and improve testing.

src/gpu/gl/GrGLGpu.cpp

Index: src/gpu/gl/GrGLGpu.cpp
diff --git a/src/gpu/gl/GrGLGpu.cpp b/src/gpu/gl/GrGLGpu.cpp
index 8033ea60e5590de4efd3236c06048bb181435183..972049aa56b36327e24671d7c1d19e3138228035 100644
--- a/src/gpu/gl/GrGLGpu.cpp
+++ b/src/gpu/gl/GrGLGpu.cpp
@@ -1951,7 +1951,8 @@ bool GrGLGpu::onGetReadPixelsInfo(GrSurface* srcSurface, int width, int height,
                                   ReadPixelTempDrawInfo* tempDrawInfo) {
     // This subclass can only read pixels from a render target. We could use glTexSubImage2D on
     // GL versions that support it but we don't today.
-    if (!srcSurface->asRenderTarget()) {
+    GrRenderTarget* srcAsRT = srcSurface->asRenderTarget();
+    if (!srcAsRT) {
         ElevateDrawPreference(drawPreference, kRequireDraw_DrawPreference);
     }
 
@@ -1992,13 +1993,20 @@ bool GrGLGpu::onGetReadPixelsInfo(GrSurface* srcSurface, int width, int height,
         tempDrawInfo->fTempSurfaceDesc.fConfig = kRGBA_8888_GrPixelConfig;
         tempDrawInfo->fSwapRAndB = true;
         ElevateDrawPreference(drawPreference, kRequireDraw_DrawPreference);
+    } else if (kRequireDraw_DrawPreference == *drawPreference &&
+               readConfig == kAlpha_8_GrPixelConfig &&
+               !this->caps()->isConfigRenderable(kAlpha_8_GrPixelConfig, false)) {
+        // On some GLs there is no renderable single channel format. In that case we fall back
+        // to rendering to a RGBA and then will extract the alpha from that.
+        if (this->caps()->isConfigRenderable(kRGBA_8888_GrPixelConfig, false)) {
+            tempDrawInfo->fTempSurfaceDesc.fConfig = kRGBA_8888_GrPixelConfig;
+        } else {
+            return false;
+        }
     }
 
-    GrRenderTarget* srcAsRT = srcSurface->asRenderTarget();
-    if (!srcAsRT) {
-        ElevateDrawPreference(drawPreference, kRequireDraw_DrawPreference);
-    } else if (read_pixels_pays_for_y_flip(srcAsRT, this->glCaps(), width, height, readConfig,
-                                           rowBytes)) {
+    if (srcAsRT && read_pixels_pays_for_y_flip(srcAsRT, this->glCaps(), width, height, readConfig,
+                                               rowBytes)) {
         ElevateDrawPreference(drawPreference, kGpuPrefersDraw_DrawPreference);
     }
 
@@ -2049,6 +2057,28 @@ bool GrGLGpu::onReadPixels(GrSurface* surface,
             SkFAIL("Unknown resolve type");
     }
 
+    // We have a special case fallback for reading alpha from a RGBA/BGRA surface. We will read
+    // back all the channels as RGBA and then extract A.
+    // This must be called after the RT is bound as the FBO above.
+    if (!this->glCaps().readPixelsSupported(this->glInterface(), config, tgt->config())) {
+        if ((tgt->config() == kRGBA_8888_GrPixelConfig ||
+             tgt->config() == kBGRA_8888_GrPixelConfig) &&
+            kAlpha_8_GrPixelConfig == config) {
+            SkAutoTDeleteArray<uint32_t> temp(new uint32_t[width * height * 4]);
+            if (this->onReadPixels(surface, left, top, width, height, kRGBA_8888_GrPixelConfig,
+                                   temp.get(), width*4)) {
+                uint8_t* dst = reinterpret_cast<uint8_t*>(buffer);
+                for (int j = 0; j < height; ++j) {
+                    for (int i = 0; i < width; ++i) {
+                        dst[j*rowBytes + i] = (0xFF000000U & temp[j*width+i]) >> 24;
+                    }
+                }
+                return true;
+            }
+        }
+        return false;
+    }
+
     const GrGLIRect& glvp = tgt->getViewport();
 
     // the read rect is viewport-relative