Use glDiscardFramebuffer to save memory bandwidth

Source/platform/graphics/gpu/DrawingBuffer.cpp

Index: Source/platform/graphics/gpu/DrawingBuffer.cpp
diff --git a/Source/platform/graphics/gpu/DrawingBuffer.cpp b/Source/platform/graphics/gpu/DrawingBuffer.cpp
index 1ef7c63f9bcf88e713336f7dc289f4f1f18816bd..4bbb1ff362a3a188a9cd3dd29268a6029137d0e0 100644
--- a/Source/platform/graphics/gpu/DrawingBuffer.cpp
+++ b/Source/platform/graphics/gpu/DrawingBuffer.cpp
@@ -264,6 +264,10 @@ bool DrawingBuffer::prepareMailbox(WebExternalTextureMailbox* outMailbox, WebExt
             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);
     }