Mumble: Upgrade Opus to 1.3

[xaro1]

https://people.xiph.org/~jm/opus/opus-1.3/

Lot of low bitrate speech improvements.

[wildxthing]

There is a newer version Opus 1.3.1 with some minor fixes.

http://opus-codec.org/release/stable/2019/04/12/libopus-1_3_1.html

[Krzmbrzl]

I assume this is more work than simply replacing the old library with the new one?

[davidebeatrici]

Updating the submodule is enough. If source files have been added/removed, we also have to update our build project:
https://github.com/mumble-voip/mumble/blob/713b92c3c233f8f38200c4a458e0196d3d39a3b0/3rdparty/opus-build/opus-build.pro#L77-L249