Revert "[rbp] Always use the aspect ratio from file."

This reverts commit 7f5d3ede514f30552e42464c29227d570b38ed8a.
[rbp/omxplayer] Use aspect ratio from video decoder

We don't get a valid aspect ratio from tvheadend with mpeg2 files, but the decoder does know, so use that.
This also supports streams where aspect ratio changes (e.g. at commercials) (with firmware from 25 June or newer).
Merge pull request #2919 from popcornmix/aspect_from_decoder

[rbp/omxplayer] Use aspect ratio from video decoder