vo_vaapi: fix reference counting when using software decoding

Recent regression.
build: remove some trailing whitespace
client API: declare as stable

It is reasonably stable, so all further changes will be versioned.

Also change how the libmpv version number is generated. Fix the patch
version number to 0; I don't think we have a use for this. In
particular, the version doesn't version mpv, just the client API.
stream_dvd: fix potential endless loop on seeking

Attempt to fix a reported freeze with some DVDs. Unknown if this helps,
and it still might read the whole DVD before terminating.