st/xlib: Fix upside down coordinates for CopySubBuffer

The coordinates need to be inverted between glX and gallium.

NOTE: This is a candidate for stable release branches.

Reviewed-by: Brian Paul <brianp@vmware.com>

src/gallium/state_trackers/glx/xlib/xm_api.c

@@ -1255,7 +1255,7 @@ void XMesaCopySubBuffer( XMesaBuffer b, int x, int y, int width, int height )
 {
    xmesa_copy_st_framebuffer(b->stfb,
          ST_ATTACHMENT_BACK_LEFT, ST_ATTACHMENT_FRONT_LEFT,
-         x, y, width, height);
+         x, b->height - y - height, width, height);
 }