x11: inline a function

Keeping it separate seems less readable.
x11: don't cache X Atoms manually

XInternAtom() already caches lookups. Even if calling XInternAtom would
be always inefficient, it wouldn't matter much during normal playback.
x11: remove gravity restore code

It was added with 3813c685 in 2004. I'm not really sure why this gravity
stuff would be needed; apparently it has to do with misplacements with
broken WMs and had to be changed on fullscreen. Just get rid of it; it
works perfectly fine without on modern WMs.

The thread discussing this is here:
http://mplayerhq.hu/pipermail/mplayer-dev-eng/2004-July/027674.html
x11: use CenterGravity by default

Keeps the window centered on resize. Seems nicer. (Although it's worse
if 1. the default placement of the WM puts it into a monitor corner,
and 2. you switch to a larger video.)
x11: implement --fs-screen properly, separate old code path

Try to get the "new" code path (using NetWM/EWMH) free of hacks done for
the sake of old WMs or the no-WM case.

Implement --fs-screen using _NET_WM_FULLSCREEN_MONITORS.
x11: always raise layer in fullscreen mode without NetWM
x11: remove vo_hint member

Now it's always recreated in vo_x11_sizehint(). Also, the Xlib manual
says you must use XAllocSizeHints() (for ABI reasons), so do that.
x11: don't set PBaseSize

There's apparently no reason why we should set a bogus size.
x11: remove some unused fields
x11: replace x/y/w/h with mp_rect