Add necessary defines and function prototypes for EXT_draw_buffers and multiple color attachment points to gl2ext.h.

TRAC #22656

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

git-svn-id: https://angleproject.googlecode.com/svn/branches/dx11proto@2003 736b8ea6-26fd-11df-bfd4-992fa37f6226