GLX/DRI2: add INTEL_swap_event support

This allows clients to easily check for swap completion status in their
main loop.

Reviewed-by: Kristian Høgsberg <krh@bitplanet.net>
Reviewed-by: Adam Jackson <ajax@nwnk.net>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>

glx/extension_string.h

@@ -50,6 +50,7 @@ enum {
    SGIX_fbconfig_bit,
    SGIX_pbuffer_bit,
    SGIX_visual_select_group_bit,
+   INTEL_swap_event_bit,
    __NUM_GLX_EXTS,
 };