The only reference to it in server and drivers is in XAA overlay code which
would segfault as no miInitOverlay is called ever. No segfaults were observed
"in wild", so XAA overlay is probably also unused.

XAA code is modified to act as if miOverlayCopyUnderlay always returned false,
because XAACopyWindow8_32 could only set doUnderlay to true if it's called from
miOverlayMoveWindow or miOverlayResizeWindow, which can only be called if
miInitOverlay has hooked those functions, and no driver (on fd.o) or server code
calls that.

Signed-off-by: Mikhail Gusarov <dottedmag@dottedmag.net>
Reviewed-by: Jamey Sharp <jamey@minilop.net>