Use glDiscardFramebuffer to save memory bandwidth

Source/platform/graphics/gpu/DrawingBuffer.cpp

 /*
  * 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 246 matching lines @@
         std::swap(frontColorBufferMailbox->textureInfo, m_colorBuffer);
         // It appears safe to overwrite the context's framebuffer binding in the Discard case since there will always be a
         // WebGLRenderingContext::clearIfComposited() call made before the next draw call which restores the framebuffer binding.
         // If this stops being true at some point, we should track the current framebuffer binding in the DrawingBuffer and restore
         // it after attaching the new back buffer here.
         m_context->bindFramebuffer(GL_FRAMEBUFFER, m_fbo);
         if (m_multisampleMode == ImplicitResolve)
             m_context->framebufferTexture2DMultisampleEXT(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, m_colorBuffer.textureId, 0, m_sampleCount);
         else
             m_context->framebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, m_colorBuffer.textureId, 0);
+
+        // Explicitly discard framebuffer to save GPU memory bandwidth for tile-based GPU arch.

[Ken Russell (switch to Gerrit), 2014/10/01 20:40:06]

Please test for the presence of this extension in DrawingBuffer::create, pass a
flag for it into the DrawingBuffer's constructor, and test the flag here before
making the call.

+        const WGC3Denum attachments[3] = { GL_COLOR_ATTACHMENT0, GL_DEPTH_ATTACHMENT, GL_STENCIL_ATTACHMENT};
+        m_context->discardFramebufferEXT(GL_FRAMEBUFFER, 3, attachments);
     } else {
         m_context->copyTextureCHROMIUM(GL_TEXTURE_2D, m_colorBuffer.textureId, frontColorBufferMailbox->textureInfo.textureId, 0, GL_RGBA, GL_UNSIGNED_BYTE);
     }
 
     if (m_multisampleMode != None && !m_framebufferBinding)
         bind();
     else
         restoreFramebufferBinding();
 
     m_contentsChanged = false;
@@ skipping 771 matching lines @@
 void DrawingBuffer::deleteChromiumImageForTexture(TextureInfo* info)
 {
     if (info->imageId) {
         m_context->releaseTexImage2DCHROMIUM(GL_TEXTURE_2D, info->imageId);
         m_context->destroyImageCHROMIUM(info->imageId);
         info->imageId = 0;
     }
 }
 
 } // namespace blink