demux_lavf: refactor format probing hack

Restructure it a bit, so we can use the format hack list even if no mime
type applies. Shouldn't change anything functionally yet. Preparation
for the next commit.
demux_lavf: add hack to workaround too unreliable mp3 detection

This generally affects mp3 files that don't have any (or many) mp3
frames in the first 2 MB. 2 MB is the maximum probe size, and
libavformat returns a low probescore even if we give it the full 2 MB.
Trying to probe a larger buffer (or even the full file) doesn't work for
mysterious reasons.

The workaround consists in accepting a very weak probescore if the
format is detected as mp3 and we probed already 2 MB.