[AD] [LNX] Mode-X and threads and mouse

[Peter Wang <tjaden@xxxxxxxxxx>]

A segfault occurs under Linux console when using mode-X and the mouse
and the mouse pointer is visible and threads are in use.  The problem
is that input is now handled in a thread, so when the mouse moves we
end up calling mode-X drawing from that thread.

Can someone take a look at modexgfx.s?  In particular, the functions
that need to be usable in this situation are _x_draw_sprite,
_x_blit_from_memory, _x_blit_to_memory.  As confirmation, no segfault
occurs if those three functions are made to return directly or aren't
called (from mouse.c:~203).