[AD] XOR drawing under X11 (and what is _xwin_in_gfx_call)

[Elias Pschernig <elias@xxxxxxxxxx>]

About that XOR bug we talked about yesterday in #allegro - I propose to
leave X11 always in solid mode. I looked some more at this in a separate
testcase, and it seems that either the mouse drawing causes the drawing
in the main thread to get solid, or the spline drawing causes the mouse
drawing to get XORed.

In any case, having to preserve the XOR state seems too much hassle (and
possibly performance loss) compared to no gain (when does XOR need to be
done by X11?) at all. Just let's have X11 draw always in solid mode,
with no state information to carry around, for now.

Ideally I'd like the X11 port to be refactored so it gets somewhat
clearer, and then something like XOR could be added again, by adding it
to one single place where it belongs to, without any possibility of side
effects. #allegro showed that Peter and Chris are almost as confused
about some internals of the X11 port as me :) For example nobody knows
what _xwin_in_gfx_call is for exactly, apparently it can be set also
with the pthread driver, so it's not just something for the signals
version.

-- 
Elias Pschernig