Adds support for EXT_framebuffer_multisample

I probably need more tests but in order to test
I need to figure out how to init the system
with different caps enabled from the unit tests
and that's probably not a small amount of work
so leaving that till later.

TEST=some unit tests, ran conformance tests
BUG=none

Review URL: http://codereview.chromium.org/3122033

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@57081 0039d316-1c4b-4281-b951-d872f2087c98