vo_opengl: don't assume there'a always 1 fbconfig on success

Seems to be a reasonable assumption, but it's probably not guaranteed.
video/out: don't access aspdat in VOs

vo->aspdat is basically an outdated version of vo->params, plus some
weirdness. Get rid of it, which will allow further cleanups and which
will make multithreading easier (less state to care about).

Also, simplify some VO code by using mp_image_set_attributes() instead
of caring about display size, colorspace, etc. manually. Add the
function osd_res_from_image_params(), which is often needed in the case
OSD renders into an image.
vo: move vo_get_src_dst_rects to aspect.c

The plan is to make all the code in aspect.c independent from vo.c,
which should make the code easier to understand, will allow removal of
vo->aspdat, and reduces the amount of code that accesses weird mutable
struct vo fields.
vo: merge get rid of vo.aspdat field

Rename vo_get_src_dst_rects() to mp_get_src_dst_rects() and make it
independent from the VO (it takes a comical amount of parameters now to
pass all required state). Add a convenience wrapper with the name
vo_get_src_dst_rects() to vo.c. Replace all aspdat and vo usages with
immediate parameters.

Functionally, nothing should change, except that the window size is
clamped to a minimum of size 1 much earlier, and some log messages
change the prefix (don't bother with vo.vo_log stuff).
aspect: remove a small ffmpeg dependency

Not strictly needed, but probably saves us pain the next time
ffmpeg mess up their headers.