vd: remove some dead code

As of now, this function is called only after decoding a full, valid
video frame, so the image parameters should be reliable.
vd: move aspect calculation to a function

This function would probably be useful in other places too.

I'm not sure why vd.c doesn't apply the aspect if it changes size by
less than 4 pixels. Maybe it's supposed to avoid ugly results with bad
scalers if the difference is too small to be noticed normally.
video: let sh_video->aspect always be container aspect ratio

Now writing -1 to the 'aspect' property resets the video to the auto
aspect ratio. Returning the aspect from the property becomes a bit more
complicated, because we still try to return the container aspect ratio
if no frame has been decoded yet.
video: let sh_video->disp_w/h always be container size

Nothing really accesses it. Subtitle initialization actually does in a
somewhat meaningful way, but there container size is probably fine, as
subtitles were always initialized before the first video frame was
decoded.
osd_libass: add "Default" dummy style

This is pretty much a hack for the OSC. It will allow it to rely on a
somewhat predictable style, instead of having to overwrite all user
OSD settings manually with override tags.