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In combination with shawnsingh's scissoring, this is a huge win on 10.5 which uses a readback through main memory. Some numbers given on https://bugs.webkit.org/show_bug.cgi?id=67341

The win on >= 10.6 is probably less because there is not readback through main memory. The 10.6 implementation assumes that nothing outside the partial swap region was damaged (i.e. it's a SwapBuffers that always preserves the backbuffer --- it's not a true partial swap a la GLX_MESA_copy_sub_buffers). This is all that the scissoring needs.

BUG=none
TEST=ran with shawnsingh's patch on http://www.webkit.org/blog-files/3d-transforms/poster-circle.html on 10.6 and 10.5

Review URL: http://codereview.chromium.org/8726046

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@113397 0039d316-1c4b-4281-b951-d872f2087c98