Add DRI3+Present loader

Add DRI3+Present loader

Uses the __DRIimage loader interfaces.

v2: Fix _XIOErrors when DRI3 isn't present (change by anholt).  Apparently
    XCB just terminates your connection if you don't check for extensions
    before using them, instead of returning an error like you'd expect.

Signed-off-by: Keith Packard <keithp@keithp.com>
Reviewed-by: Kristian Høgsberg <krh@bitplanet.net>
Reviewed-by: Eric Anholt <eric@anholt.net>