version.sh: integrate the release tag in output

This is basically reconstructed from 46b218c. Since we now have proper
release tags, we want this again.

Add --tags to the git describe call, because the github release system
creates light-weight tags only, and we're too lazy to create annitated
tags (or is that bad practice?).

Add --long, so that the git commit hash is part of the output even if
the tag matches.
version.sh: add --print option, which prints version to stdout

Might be helpful when creating a tarball.
core: make sure hw decoding works when playing multiple files

The hw decoding context was set only for the first file (when the VO was
initialized), instead of every file.
vo: unref waiting image before destroying VO driver

This is in theory more correct with respect to hardware decoding. With
hardware decoding, the VOs play the role of the video surface allocator,
and nothing is allowed to reference surfaces past the VO lifetime. But
in theory waiting_mpi could be a reference to a HW video surface, so it
should be relased before the VO is uninitialized.