Fix BGRA/RGBA readback conversions on mac

src/gpu/gl/GrGLCaps.h

Index: src/gpu/gl/GrGLCaps.h
diff --git a/src/gpu/gl/GrGLCaps.h b/src/gpu/gl/GrGLCaps.h
index 06deaafae10676159c479a0a1abb21250c10e8e4..63a3793eede9e7e55141a1b367932ef53fe9d5cd 100644
--- a/src/gpu/gl/GrGLCaps.h
+++ b/src/gpu/gl/GrGLCaps.h
@@ -351,6 +351,9 @@ public:
 
     bool rgba8888PixelsOpsAreSlow() const { return fRGBA8888PixelsOpsAreSlow; }
     bool partialFBOReadIsSlow() const { return fPartialFBOReadIsSlow; }
+    bool rgbaToBgraReadbackConversionsAreSlow() const {
+        return fRGBAToBGRAReadbackConversionsAreSlow;
+    }
 
     const GrGLSLCaps* glslCaps() const { return reinterpret_cast<GrGLSLCaps*>(fShaderCaps.get()); }
 
@@ -421,6 +424,7 @@ private:
     bool fRectangleTextureSupport : 1;
     bool fTextureSwizzleSupport : 1;
     bool fMipMapLevelAndLodControlSupport : 1;
+    bool fRGBAToBGRAReadbackConversionsAreSlow : 1;
 
     BlitFramebufferSupport fBlitFramebufferSupport;