Some ATI cards appear not to flush the pipeline correctly, causing textures not to pick up newly-drawn contexts, so do an extra no-op draw when necessary.

BUG=169
TEST=chromium on hulu.com

Review URL: http://codereview.appspot.com/4576051

git-svn-id: https://angleproject.googlecode.com/svn/trunk@693 736b8ea6-26fd-11df-bfd4-992fa37f6226