video: add rounding to aspect ratio calculations

Small errors are unavoidable, but truncation can cause anamorphic video
to be off by 1 or 2 pixels.
input: minor simplification for --input-test
input, dvdnav: fix osc stealing input from dvdnav

This is a regression introduced from moving Lua scripts (including the
OSC) to their own threads. Now OSC and dvdnav can add their bindings at
the same time without coordination, which seems to result in the OSC
winning most time, and thus overriding the dvdnav menu bindings.

Fix this by adding a flag that makes dvdnav menu bindings take priority
over all other bindings.
client API: add event for metadata changes
manpage: lua: improve explanation of tracks-changed and tracks-switched
command: move metadata entry access to metadata/by-key/

The old way still works, and is fine to use. Still discourage it,
because it might conflict with other ways to access this property, such
as the one added in the next commit.
command: allow accessing metadata entries as list

Not sure about these deep path-names. Maybe "metadata/0" should work
instead of "metadata/list/0". I'm so unsure about it, that I'm leaving
it open.
manpage: input: clarify chapter-metadata property