sub: add webvtt-in-webm support

The way this was added to FFmpeg is less than ideal, because it requires
text parsing in the Matroska demuxer. But in order to use the FFmpeg
webvtt-to-ass converter, we still have to mimic this in some way. We do
this by putting the parsing into sd_lavc_conv.c, before the subtitle
packet is passed to libavcodec. At least this keeps the ugliness out of
unrelated code.

There is some change that FFmpeg will fix their design eventually.

Instead of rewriting the parsing code, we simply borrow it from FFmpeg's
Matroska demuxer.
vo_vaapi: potentially fix screenshot colorspace issues

mp_image_set_params() doesn't check whether the colorspace parameters
are  consistent (e.g. setting YUV colorspaces with RGB formats), and
shouldn't need to.
video/out: don't require VOs to handle screenshot aspect specially

This affects VOs which just reuse the mp_image from draw_image() to
return screenshots. The aspect of these images is never different
from the aspect the screenshots should be, so there's no reason to
adjust the aspect in these cases.

Other VOs still need it in order to restore the original image
attributes.

This requires some changes to the video filter code to make sure that
the aspect in the passed mp_images is consistent.

The changes in mplayer.c and vd_lavc.c are (probably) not strictly
needed for this commit, but contribute to consistency.
vo_image: simplify
video: handle video output levels with mp_image_params

Until now, video output levels (obscure feature, like using TV screens
that require RGB output in limited range, similar to YUY) still required
handling of VOCTRL_SET_YUV_COLORSPACE. Simplify this, and use the new
mp_image_params code. This gets rid of some code. VOCTRL_SET_YUV_COLORSPACE
is not needed at all anymore in VOs that use the reconfig callback. The
result of VOCTRL_GET_YUV_COLORSPACE is now used only used for the
colormatrix related properties (basically, for display on OSD).  For
other VOs, VOCTRL_SET_YUV_COLORSPACE will be sent only once after config
instead of twice.
configure: fix VDA warning on systems other than OSX

CONFIG_VDA is supposed to be defined to 0 or 1. But on non-OSX systems,
the configure test isn't run at all, so CONFIG_VDA ends up undefined.