Working on assimp integration

Yop, I’m adding the Open Asset Import Library as a fallback for any model formats that aren’t natively supported with their own dedicated loader (only ones with dedicated loaders are glTF2, PLY, STL and OBJ). I’m still keeping the dedicated loaders, because they are more feature-complete, to-spec and wayyy faster (especially the glTF one).

I added it because I feel like for a 3D model viewer, support for many formats, even niche and obscure ones, is pretty important, even though nobody might use them.

This roughly doubled compile time and almost quadrupled binary size, which also made CI take way longer (of course). It somehow made the MacOS runners take 50 minutes to compile (from the previous 3 minutes), but I fixed that by bounding the number of processes CMake can spawn to the number of threads on the runner (previously was using -j which spawns as many as it wants and it was spawning about 230 of them).

I’m still testing how it handles various model formats and trying to find any reachable segfaults.