make alpha read back of bgra/rgba work

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 fdf7eece7efb6f38b4d28e00a9cd4958bc00c887..d07203e376baba7dd0da29dddf0247ca0c1d5add 100644
--- a/src/gpu/gl/GrGLGpu.cpp
+++ b/src/gpu/gl/GrGLGpu.cpp
@@ -1594,7 +1594,7 @@ bool GrGLGpu::flushGLState(const DrawArgs& args) {
     }
 
     if (blendInfo.fWriteColor) {
-        this->flushBlend(blendInfo);
+        this->flushBlend(blendInfo, SkToBool(args.fDesc->header().fSwapDstRedAndAlpha));
     }
 
     SkSTArray<8, const GrTextureAccess*> textureAccesses;
@@ -2073,6 +2073,30 @@ bool GrGLGpu::onReadPixels(GrSurface* surface,
             SkFAIL("Unknown resolve type");
     }
 
+    // This must be called after the RT is bound as the FBO above.
+    if (!this->glCaps().readPixelsSupported(this->glInterface(), format, type, tgt->config())) {
+        // We fail except the special case of reading back just the alpha channel of a RGBA/BGRA
+        // target.
+        if ((tgt->config() == kRGBA_8888_GrPixelConfig ||
+             tgt->config() == kBGRA_8888_GrPixelConfig) &&
+            kAlpha_8_GrPixelConfig == config) {
+            SkDebugf("");
+            SkAutoTDeleteArray<uint32_t> temp(new uint32_t[width * height * 4]);
+            if (this->onReadPixels(surface, left, top, width, height, tgt->config(), 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;
+                    }
+                }
+                SkDebugf("rowBytes: %d\n", rowBytes);
+                return true;
+            }            
+        }
+        return false;
+    }
+
     const GrGLIRect& glvp = tgt->getViewport();
 
     // the read rect is viewport-relative
@@ -2422,7 +2446,7 @@ void GrGLGpu::flushHWAAState(GrRenderTarget* rt, bool useHWAA) {
     }
 }
 
-void GrGLGpu::flushBlend(const GrXferProcessor::BlendInfo& blendInfo) {
+void GrGLGpu::flushBlend(const GrXferProcessor::BlendInfo& blendInfo, bool swapConstRedAndAlpha) {
     // Any optimization to disable blending should have already been applied and
     // tweaked the equation to "add" or "subtract", and the coeffs to (1, 0).
 
@@ -2476,15 +2500,18 @@ void GrGLGpu::flushBlend(const GrXferProcessor::BlendInfo& blendInfo) {
     }
 
     GrColor blendConst = blendInfo.fBlendConstant;
-    if ((BlendCoeffReferencesConstant(srcCoeff) ||
-         BlendCoeffReferencesConstant(dstCoeff)) &&
-        (!fHWBlendState.fConstColorValid ||
-         fHWBlendState.fConstColor != blendConst)) {
-        GrGLfloat c[4];
-        GrColorToRGBAFloat(blendConst, c);
-        GL_CALL(BlendColor(c[0], c[1], c[2], c[3]));
-        fHWBlendState.fConstColor = blendConst;
-        fHWBlendState.fConstColorValid = true;
+    if ((BlendCoeffReferencesConstant(srcCoeff) || BlendCoeffReferencesConstant(dstCoeff))) {
+        if (swapConstRedAndAlpha) {
+            blendConst = GrColorPackRGBA(GrColorUnpackA(blendConst), GrColorUnpackG(blendConst),
+                                         GrColorUnpackB(blendConst), GrColorUnpackR(blendConst));
+        }
+        if (!fHWBlendState.fConstColorValid || fHWBlendState.fConstColor != blendConst) {
+            GrGLfloat c[4];
+            GrColorToRGBAFloat(blendConst, c);
+            GL_CALL(BlendColor(c[0], c[1], c[2], c[3]));
+            fHWBlendState.fConstColor = blendConst;
+            fHWBlendState.fConstColorValid = true;
+        }
     }
 }
 
@@ -3330,7 +3357,7 @@ void GrGLGpu::drawDebugWireRect(GrRenderTarget* rt, const SkIRect& rect, GrColor
 
     GrXferProcessor::BlendInfo blendInfo;
     blendInfo.reset();
-    this->flushBlend(blendInfo);
+    this->flushBlend(blendInfo, false);
     this->flushColorWrite(true);
     this->flushDrawFace(GrPipelineBuilder::kBoth_DrawFace);
     this->flushHWAAState(glRT, false);
@@ -3401,7 +3428,7 @@ void GrGLGpu::copySurfaceAsDraw(GrSurface* dst,
 
     GrXferProcessor::BlendInfo blendInfo;
     blendInfo.reset();
-    this->flushBlend(blendInfo);
+    this->flushBlend(blendInfo, false);
     this->flushColorWrite(true);
     this->flushDrawFace(GrPipelineBuilder::kBoth_DrawFace);
     this->flushHWAAState(dstRT, false);