engine: bail out of voxcache_fetchvoxmodel() if the offsets found in texturecache.index don't lead to the expected data

engine: don't attempt to tear down ImGui in destroy_window_resources() if we didn't even make it far enough through initialization to get it set up

This worked with all of the voxels I tested it with, but caused issues with some in Alien Armageddon.

engine: enable mimalloc abandoned_page_reset option and call mi_collect() during enginePrepareLoadBoard()

The rationale here is that it's better to provide the build instructions for these libraries (as we already do) with no binary than it is to provide outdated binaries that don't actually match the provided headers anymore.

engine: fix issue where sometimes the window didn't think it was focused at startup until focus was lost and regained

engine: rename win_priorityclass cvar to win_boostpriority and make the accepted range of values a bit saner

We were only using this to make clang behave like GCC, but some versions of GCC are bugged such that enabling -frounding-math prevents them from correctly handling constexpr float values determined at compile time.

-Missing mip levels are now drawn by scaling the highest mip level present in the file, not just taking mip 0 when e.g we want mip 4 but 0-3 are available
-Algorithm for determining which mip level to use has been adjusted to bias for rendering quality and not just speed (e.g don't display the chunkier mip levels at such close distances)
-Mip levels other than the default are now run through a conversion process that standardizes the colors used based on those found in mip 0, to work around issues where the mip levels ended up with fullbright colors substituted in place of regular palette entries when the mips were originally generated by SLABSPRI/SLAB6
-Add terrible heuristic for mip level rejection based on how many visible voxels are present in the higher mip level versus the lower mip levels for the same voxel. This is intended to work around issues where SLABSPRI and SLAB6 both generated their mips by removing cubes a bit too liberally, resulting in any feature of the model that was only 1 voxel thick being completely obliterated by the mip generation process. This can be observed by setting r_novoxmips to 0 in older builds and viewing 8756_FLOORLAMP_ON.kvx from Ion Fury 2.0 or earlier at various distances in-game. Without this heuristic, poorly generated mip levels that end up obliterating the entire lampshade are drawn.

Not particularly exciting or useful now that the converted versions are cached.

Fixes textures with fullbright pixels being drawn completely bright when texture filtering is enabled, and a couple other issues.

Makefile: rework to use exclusion lists instead of inclusion lists for determining which objects to use as part of the build

Benefits: almost 200 lines deleted from the Makefile, new source files added to the project require no additional setup.
Drawback: you can't leave random shit with .c or .cpp file extensions laying around your source directories (sounds more like another benefit tbh).

engine: clipping change: attempt to apply 5f50a454 to sprites and not just walls

Something somewhere is still fucked, though (and was before this commit)

engine: fix bug where sometimes samplers would still end up bound to texture units after render mode switches

audiolib: add useCallBack parameter to FX_StopSound() and FX_StopAllSounds() (and, by extension, MV_Kill() and MV_KillAllVoices())