Backport some fixes on 1.x

[robUx4]

A lot of things are being done in 2.x. Some of the changes can be merged in 1.x, like

• a7972aa

We can do that once the dust has settled.

[mbunkus]

To be quite honest, I'm not that interested in spending more effort or v1 apart from obvious bug fixes/security issues. I plan to remove v1 support in MKVToolNix as soon as v2 comes out. That'll get rid of quite some compatibility code. Drawback is that my means of testing such backports will be greatly reduced.