Remove the old single-color-attachment calls from gl::Framebuffer, preferring the new multiple-attachment versions.

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@2008 736b8ea6-26fd-11df-bfd4-992fa37f6226