Patch removing duplicated code in the setup of the DrawingBuffer used

Patch removing duplicated code in the setup of the DrawingBuffer used
to host the back-buffer for WebGL contents.
https://bugs.webkit.org/show_bug.cgi?id=72327

Patch by Jeff Timanus <twiz@chromium.org> on 2011-11-15
Reviewed by Kenneth Russell.

* html/canvas/WebGLRenderingContext.cpp:
(WebCore::WebGLRenderingContext::copyTexImage2D):
(WebCore::WebGLRenderingContext::copyTexSubImage2D):
(WebCore::WebGLRenderingContext::readPixels):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@100399 268f45cc-cd09-0410-ab3c-d52691b4dbfc