semantic for x and y coordinates rather than previous per-vertex
emulation when running on Shader Model 3.

Verified fix in Chromium with test case from bug.

BUG=http://code.google.com/p/angleproject/issues/detail?id=71
TEST=test case from bug
TBR=nicolas (LGTM'd by dgkoch)

Review URL: http://codereview.appspot.com/2921041

git-svn-id: https://angleproject.googlecode.com/svn/trunk@473 736b8ea6-26fd-11df-bfd4-992fa37f6226