Enable broadcasting gl_FragColor to all draw buffer color attachments in ES2 contexts.

TRAC #22888

Signed-off-by: Nicolas Capens
Signed-off-by: Shannon Woods
Author: Jamie Madill

git-svn-id: http://angleproject.googlecode.com/svn/branches/dx11proto@2052 736b8ea6-26fd-11df-bfd4-992fa37f6226