Updates to MRT extension.

Trac #21120

    01/30/2013  dgkoch  add issue 6 and clear interactions
                        renamed to EXT_draw_buffers based on feedback
                        changed resolution of issue 3.

git-svn-id: http://angleproject.googlecode.com/svn/trunk@1806 736b8ea6-26fd-11df-bfd4-992fa37f6226
Rename ANGLE_multiple_render_targets to EXT_draw_buffers

Trac #21120

git-svn-id: http://angleproject.googlecode.com/svn/trunk@1807 736b8ea6-26fd-11df-bfd4-992fa37f6226