Make brush surfaces loading uniform

[w23]

Currently brush model loading consists of the following steps:

1. Go through all its surfaces and compute counts of each type of surface.
2. Load regular brush model. This also iterates through all surfaces, gets their type, etc etc. Also determines and handles whether that surface is emissive.
3. Load water and water sides model. Does some of that processing too.

There's a bunch of duplicate code, and unnatural similar argument passing around.

At this scale it would make sense to consolidate surface processing:

1. Loop over all surfaces only once.
2. Compute common parameters for each one.
3. Handle emissive for each one independent of type, see e.g. Emissive textures on !water textures does not work #56.
4. Do specific handling based on type, e.g.: regular types go to regular submodel, water/sides go to water submodels, etc.
5. When done with surfaces, finalize each submodel: create render models, upload geometries, etc.

Note that for efficient model loading we might need to compute vertices+indices count before allocating geometry buffers. That might either need a second pass over loaded surfaces for each submodel type, or using dynamic arrays for temp data.

Doing an explicit finalization step could also allow for more natural extended geometry processing, e.g.:

• Displacement map (tesselation) #164
• optimizing polygon lights: joining bsp splits, performing retopology for more efficient sampling, etc.