GL_ANGLE_multiple_render_targets in order to match the GL extension
name. Using XYZ_draw_buffers as in GL_ARB_draw_buffers and
GL_NV_draw_buffers was causing inconsistencies.
Review URL: https://codereview.appspot.com/6715045

git-svn-id: https://angleproject.googlecode.com/svn/trunk@1293 736b8ea6-26fd-11df-bfd4-992fa37f6226