mesa: add GL_OES/EXT_draw_buffers_indexed support

This is the same ext as ARB_draw_buffers_blend (plus some core
functionality that already exists). Add the alias entrypoints.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Dave Airlie <airlied@redhat.com>
diff --git a/docs/GL3.txt b/docs/GL3.txt
index c76b1e2..5b6dc89 100644
--- a/docs/GL3.txt
+++ b/docs/GL3.txt
@@ -257,7 +257,7 @@
   GL_KHR_robustness                                     not started (90% done with the ARB variant)
   GL_KHR_texture_compression_astc_ldr                   DONE (i965/gen9+)
   GL_OES_copy_image                                     DONE (core only)
-  GL_OES_draw_buffers_indexed                           not started
+  GL_OES_draw_buffers_indexed                           DONE (all drivers that support GL_ARB_draw_buffers_blend)
   GL_OES_draw_elements_base_vertex                      DONE (all drivers)
   GL_OES_geometry_shader                                started (Marta)
   GL_OES_gpu_shader5                                    DONE (all drivers that support GL_ARB_gpu_shader5)