egl: Inform the client API when ancillary buffers may become undefined.

This is part of the EGL spec, and is useful for a tiled renderer to avoid
the memory bandwidth cost of storing the depth/stencil buffers.

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>

include/GL/internal/dri_interface.h

@@ -279,6 +279,7 @@ struct __DRItexBufferExtensionRec {
 
 #define __DRI2_FLUSH_DRAWABLE (1 << 0) /* the drawable should be flushed. */
 #define __DRI2_FLUSH_CONTEXT  (1 << 1) /* glFlush should be called */
+#define __DRI2_FLUSH_INVALIDATE_ANCILLARY (1 << 2)
 
 enum __DRI2throttleReason {
    __DRI2_THROTTLE_SWAPBUFFER,