present: Add Present extension

Provides both a software implementation using timers and driver hooks
to base everything on vblank intervals.

Signed-off-by: Keith Packard <keithp@keithp.com>
Reviewed-by: Adam Jackson <ajax@redhat.com>

include/extinit.h

@@ -181,4 +181,9 @@ extern void XvMCExtensionInit(void);
 extern void dri3_extension_init(void);
 #endif
 
+#if defined(PRESENT)
+#include <X11/extensions/presentproto.h>
+#include "presentext.h"
+#endif
+
 #endif