Use glDiscardFramebuffer to save memory bandwidth

Source/platform/graphics/gpu/DrawingBuffer.h

 /*
  * Copyright (c) 2010, Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 118 matching lines @@
     void setFramebufferBinding(Platform3DObject fbo) { m_framebufferBinding = fbo; }
 
     // Track the currently active texture unit. Texture unit 0 is used as host for a scratch
     // texture.
     void setActiveTextureUnit(GLint textureUnit) { m_activeTextureUnit = textureUnit; }
 
     bool multisample() const;
 
     Platform3DObject framebuffer() const;
 
+    bool discardFramebufferSupported() const { return m_discardFramebufferSupported; }
+
     void markContentsChanged();
     void markLayerComposited();
     bool layerComposited() const;
     void setIsHidden(bool);
 
     WebLayer* platformLayer();
     void paintCompositedResultsToCanvas(ImageBuffer*);
 
     WebGraphicsContext3D* context();
 
@@ skipping 13 matching lines @@
 
     void paintRenderingResultsToCanvas(ImageBuffer*);
     PassRefPtr<Uint8ClampedArray> paintRenderingResultsToImageData(int&, int&);
 
 protected: // For unittests
     DrawingBuffer(
         PassOwnPtr<WebGraphicsContext3D>,
         PassOwnPtr<Extensions3DUtil>,
         bool multisampleExtensionSupported,
         bool packedDepthStencilExtensionSupported,
+        bool discardFramebufferSupported,
         PreserveDrawingBuffer,
         WebGraphicsContext3D::Attributes requestedAttributes,
         PassRefPtr<ContextEvictionManager>);
 
     bool initialize(const IntSize&);
 
 private:
     void mailboxReleasedWithoutRecycling(const WebExternalTextureMailbox&);
 
     unsigned createColorTexture();
@@ skipping 50 matching lines @@
     Platform3DObject m_texture2DBinding;
     Platform3DObject m_framebufferBinding;
     GLenum m_activeTextureUnit;
 
     OwnPtr<WebGraphicsContext3D> m_context;
     OwnPtr<Extensions3DUtil> m_extensionsUtil;
     IntSize m_size;
     WebGraphicsContext3D::Attributes m_requestedAttributes;
     bool m_multisampleExtensionSupported;
     bool m_packedDepthStencilExtensionSupported;
+    bool m_discardFramebufferSupported;
     Platform3DObject m_fbo;
     // DrawingBuffer's output is double-buffered. m_colorBuffer is the back buffer.
     TextureInfo m_colorBuffer;
     TextureInfo m_frontColorBuffer;
 
     // This is used when we have OES_packed_depth_stencil.
     Platform3DObject m_depthStencilBuffer;
 
     // These are used when we don't.
     Platform3DObject m_depthBuffer;
@@ skipping 44 matching lines @@
     // used to resize the Canvas.
     SkBitmap m_resizingBitmap;
 
     // Used to flip a bitmap vertically.
     Vector<uint8_t> m_scanline;
 };
 
 } // namespace blink
 
 #endif // DrawingBuffer_h