player: cosmetics: rename a function

Something which has this many important sideffects shouldn't start have
a "get" prefix.
player: fix random crashes on uninitialization

On uninitialization, the player will unselect all subtitles, and then
destroy the subtitle decoder. But it didn't correctly remove the
subtitle decoder from the OSD state, so it could happen that it would
access it after the decoder was destroyed.

Could lead to random crashes when switching files often.

Fixes #1389.
player: force stereo as default when encoding

I think the new default for this option might be rather bad in encoding
mode. For starters, we don't even know what layers an encoder supports
at all.