Add ability to query read pixels support without a render target.

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 aeb58659bc58bd4c400ba67634fa47a282044958..044ecce81438c5f67494b008abd16e890baeb8a4 100644
--- a/src/gpu/gl/GrGLCaps.h
+++ b/src/gpu/gl/GrGLCaps.h
@@ -286,11 +286,11 @@ public:
     /// Use indices or vertices in CPU arrays rather than VBOs for dynamic content.
     bool useNonVBOVertexAndIndexDynamicData() const { return fUseNonVBOVertexAndIndexDynamicData; }
 
-    /// Does ReadPixels support the provided format/type combo?
-    bool readPixelsSupported(GrGLRenderTarget* target,
+    /// Does ReadPixels support reading readConfig pixels from a FBO that is renderTargetConfig?
+    bool readPixelsSupported(GrPixelConfig renderTargetConfig,
                              GrPixelConfig readConfig,
                              std::function<void (GrGLenum, GrGLint*)> getIntegerv,
-                             std::function<void (GrGLRenderTarget*)> bindRenderTarget) const;
+                             std::function<bool ()> bindRenderTarget) const;
 
     bool isCoreProfile() const { return fIsCoreProfile; }