Draft: Include Krita's bespoke MLT plugin in our source tree.

We use a custom MLT "producer" to drive animation playback when audio is attached.

Previously this was shipped with our own fork of MLT, but this patch simplifies things by including it in our own source tree. Hopefully doing this (on top of upstreaming what we can) means that we can avoid maintaining our own full fork of MLT.

!This MR currently doesn't build due to a CLAMP macro naming conflict between Krita and MLT!