Fix BGRA/RGBA readback conversions on mac

src/gpu/gl/GrGLGpu.cpp

 /*
  * Copyright 2011 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "GrGLGpu.h"
 #include "GrGLGLSL.h"
 #include "GrGLStencilAttachment.h"
@@ skipping 2542 matching lines @@
         // For now keep assuming the draw is not a format transformation, just a draw to get to a
         // RT. We may add additional transformations below.
         ElevateDrawPreference(drawPreference, kRequireDraw_DrawPreference);
     }
     if (this->glCaps().rgba8888PixelsOpsAreSlow() && kRGBA_8888_GrPixelConfig == readConfig &&
         this->readPixelsSupported(kBGRA_8888_GrPixelConfig, kBGRA_8888_GrPixelConfig)) {
         tempDrawInfo->fTempSurfaceDesc.fConfig = kBGRA_8888_GrPixelConfig;
         tempDrawInfo->fSwizzle = GrSwizzle::BGRA();
         tempDrawInfo->fReadConfig = kBGRA_8888_GrPixelConfig;
         ElevateDrawPreference(drawPreference, kGpuPrefersDraw_DrawPreference);
-    } else if (kMesa_GrGLDriver == this->glContext().driver() &&
+    } else if (this->glCaps().rgbaToBgraReadbackConversionsAreSlow() &&
                GrBytesPerPixel(readConfig) == 4 &&
                GrPixelConfigSwapRAndB(readConfig) == srcConfig &&
                this->readPixelsSupported(srcSurface, srcConfig)) {
         // Mesa 3D takes a slow path on when reading back BGRA from an RGBA surface and vice-versa.

[reed1, 2016/03/12 11:06:36]

Possibly we should update the comment to refer to the flag, rather than Mesa
explicitly?

         // Better to do a draw with a R/B swap and then read as the original config.
         tempDrawInfo->fTempSurfaceDesc.fConfig = srcConfig;
         tempDrawInfo->fSwizzle = GrSwizzle::BGRA();
         tempDrawInfo->fReadConfig = srcConfig;
         ElevateDrawPreference(drawPreference, kGpuPrefersDraw_DrawPreference);
     } else if (!this->readPixelsSupported(srcSurface, readConfig)) {
         if (readConfig == kBGRA_8888_GrPixelConfig &&
             this->glCaps().isConfigRenderable(kRGBA_8888_GrPixelConfig, false) &&
             this->readPixelsSupported(kRGBA_8888_GrPixelConfig, kRGBA_8888_GrPixelConfig)) {
             // We're trying to read BGRA but it's not supported. If RGBA is renderable and
@@ skipping 1774 matching lines @@
         if (GR_GL_TEXTURE_EXTERNAL == glTexture->target() ||
             GR_GL_TEXTURE_RECTANGLE == glTexture->target()) {
             copyParams->fFilter = GrTextureParams::kNone_FilterMode;
             copyParams->fWidth = texture->width();
             copyParams->fHeight = texture->height();
             return true;
         }
     }
     return false;
 }