video/out: remove fallback handling for screensize/monitor pixel aspect

When using --monitoraspect, but either the screen width or height or
both are unknown, a fallback is applied. This is a completely useless
obscure corner case that's going to help nobody, so get it out of the
way.
video/out: simplify monitor aspect handling

For some reason, this made all VO backends both set the screen
resolution in opts->screenwidth/height, and call
aspect_save_screenres(). Remove the latter. Move the code to calculate
the PAR-corrected window size from aspect.c to vo.c, and make it so that
the monitor PAR is recalculated when it makes sense.
video/out: remove pointless x/y parameter from vo_x11_config_vo_window

This never made any real sense; the "backend" has to access vo->dx/dy
anyway.
options: remove --screenw and --screenh

Doesn't make any sense anymore. X11 (which was mentioned in the manpage)
autodetects it, and everything else ignored the option values.

Since for incomprehensible reasons the backends and vo.c still need to
exchange information about the screensize using the option fields,
they're not removed yet.