Add class GrGLTextureRenderTarget for GL texture/rendertarget objects

include/gpu/GrSurface.h

Index: include/gpu/GrSurface.h
diff --git a/include/gpu/GrSurface.h b/include/gpu/GrSurface.h
index 9a76d3ab4f8fdb7b6aaa4485d5c2077797000b3d..9d891493b00f9f3e97fc64f0c1e7d5948d613433 100644
--- a/include/gpu/GrSurface.h
+++ b/include/gpu/GrSurface.h
@@ -24,15 +24,11 @@ public:
 
     /**
      * Retrieves the width of the surface.
-     *
-     * @return the width in texels
      */
     int width() const { return fDesc.fWidth; }
 
     /**
      * Retrieves the height of the surface.
-     *
-     * @return the height in texels
      */
     int height() const { return fDesc.fHeight; }
 
@@ -63,14 +59,14 @@ public:
     /**
      * @return the texture associated with the surface, may be NULL.
      */
-    virtual GrTexture* asTexture() = 0;
-    virtual const GrTexture* asTexture() const = 0;
+    virtual GrTexture* asTexture() { return NULL; }
+    virtual const GrTexture* asTexture() const { return NULL; }
 
     /**
      * @return the render target underlying this surface, may be NULL.
      */
-    virtual GrRenderTarget* asRenderTarget() = 0;
-    virtual const GrRenderTarget* asRenderTarget() const = 0;
+    virtual GrRenderTarget* asRenderTarget() { return NULL; }
+    virtual const GrRenderTarget* asRenderTarget() const { return NULL; }
 
     /**
      * Reads a rectangle of pixels from the surface.