x11: wait until the window is mapped

This blocks everything, until the window is actually reported as mapped.

This fixes the race condition between VO initialization and mapping the
window, which resulted in possibly different window sizes, leading to an
immediate redraw, visible as flashing.

Note that if the map event never comes for some reason, we're out of
luck and will block forever.
x11: clear window on map

vo_x11_map_window() was attempting to clear the window on map. However,
it did so immediately after the map request. It probably assumed that
the drawing calls for clearing the window would be queued along with the
map request, and then executed in the right order. However, this
assumption was wrong - the map request first has to go to the window
manager (I guess?), so a lot of things happen before the window is even
mapped.

Fix this by moving the call to the MapNotify message handler, when the
window (apparently) becomes really visible.

I also tried to set CWBackPixel to black instead, but this seemed to
result in flickering on manual resizing.
x11: set the fullscreen state before mapping the window

This should get rid of some flickering. Since this actually skips all
the wacky fullscreening code on startup, this might lead to certain
wacky features to stop working. In this case, you'll have to use the
--x11-fstype option, and disable _NETWM_STATE_FULLSCREEN usage.