video: rename VOCTRL_GET_WINDOW_SIZE

Make it clear that this accesses the un-fullscreened window size.
build: handle insane libavcodec API bullshit

The oldest supported FFmpeg release doesn't provide
av_vdpau_alloc_context(). With these versions, the application has no
other choice than to hard code the size of AVVDPAUContext. (On the other
hand, there's av_alloc_vdpaucontext(), which does the same thing, but is
FFmpeg specific - not sure if it was available early enough, so I'm not
touching it.)

Newer FFmpeg and Libav releases require you to call this function, for
ABI compatibility reasons. It's the typcal lakc of foresight that make
FFmpeg APIs terrible. mpv successfully pretended that this crap didn't
exist (ABI compat. is near impossible to reach anyway) - but it appears
newer developments in Libav change the function from initializing the
struct with all-zeros to something else, and mpv vdpau decoding would
stop working as soon as this new work is relewased.

So, add a configure test (sigh).

CC: @mpv-player/stable
cosmetics: remove a stray ';'
audio: fix obscure audio resync failure with timelines

Somehow, there was a larger misunderstanding in the code: ao_buffer
does not need to be preserved over audio reinit for proper support of
gapless audio. The actual AO internal buffer takes care of this.

In fact, preserving ao_buffer just breaks audio resync. In the ordered
chapter case, end_pts is used, which means not all audio data in the
buffer is played, thus some data is left over when audio decoding
resumes on the next segment. This triggers some code that aborts resync
if there's "audio decoded" (ao_buffer contains something), but no PTS
is known (nothing was actually decoded yet).

Simplify, and always bind the output buffer to the decoder.

CC: @mpv-player/stable (maybe)
audio: make buffer size configurable

Really only for testing.
malloc+memset(0) to calloc

Signed-off-by: wm4 <wm4@nowhere>