glx: add support for a reallyFlush() function before swap occurs.

include/GL/internal/dri_interface.h

@@ -78,6 +78,7 @@ typedef struct __DRIswrastExtensionRec		__DRIswrastExtension;
 typedef struct __DRIbufferRec			__DRIbuffer;
 typedef struct __DRIdri2ExtensionRec		__DRIdri2Extension;
 typedef struct __DRIdri2LoaderExtensionRec	__DRIdri2LoaderExtension;
+typedef struct __DRI2flushExtensionRec	__DRI2flushExtension;
 
 /*@}*/
 
@@ -245,6 +246,16 @@ struct __DRItexBufferExtensionRec {
 			 __DRIdrawable *pDraw);
 };
 
+/**
+ * Used by drivers that implement DRI2
+ */
+#define __DRI2_FLUSH "DRI2_Flush"
+#define __DRI2_FLUSH_VERSION 1
+struct __DRI2flushExtensionRec {
+    __DRIextension base;
+    void (*flush)(__DRIdrawable *drawable);
+};
+
 
 /**
  * XML document describing the configuration options supported by the