vo_x11: use generic aspect ratio code, refactor

Instead of manually calculating aspect ratio etc., use
vo_get_src_dst_rects(). This has the advantage that we can drop some
special cases in aspect.c later.

Simplify the code, and move resizing the X images from draw_image() to a
new resize() function.

Do some other simplifications. The gXErrorFlag check was actually dead
code, even in mplayer-svn. Special-casing on WinID is not needed
anymore (we created an embedded window, and its size is updated by
x11_common.c).
vo_opengl_old: use generic aspect ratio code

Make sure scaled_osd still works. Although this is a completely
worthless feature for a deprecated VO.
vo_corevideo: use generic aspect ratio code

The rescaling is rather silly. vo_get_src_dst_rects() doesn't return an
uncropped image, so the texture coordinates have to be recalculated,
which looks more complicated, but is actually what the other OpenGL VOs
also do.

Tested and fixed by Stefano Pigozzi.
vo_x11: accept all swscale formats

There's no reason why there should be a separate vf_scale stage.
vo_xv: minor simplifications

The draw_osd change is a bit tricky: I guess originally, there was some
intention not to second-guess the generic aspect code, but the result is
the same and the code is more confusing.
video: simplify aspect calculation stuff

Remove lots of weird logic and dead code.

The only difference is that when specifying a monitor aspect ratio, it
will always upscale and never downscale.
video: remove aspect.h includes for files which don't need it
vo_corevideo: simplify the new code