Really, this won't make all that much difference because alias models
with more than one skin are quite rare, and those with animated skin
groups are even rarer. However, for those models that do have more than
one skin, it will allow for reduced allocation overheads, and when
supported (glsl, vulkan, maybe gl), loading all the skins into an array
texture (since all skins are the same size, though external skins may
vary), but that's not implemented yet, this just wraps the old one skin
at a time code.
While the scheme of using our own allocated did work just fine, fisheye
rendering uses glGenTextures which caused a texture id clash and thus
invalid operations (the cube map texture happened to be the same as the
console background texture). Sure, I could have just "fixed" the fisheye
init code, but this brings gl closer in line with glsl (which makes
extensive use of glGenTextures and glDeleteTextures). This doesn't fix
any texture leaks gl has (plenty, I imagine), but it's a step in the
right direction.
As much as it can be since the texture data is interleaved with the
model data in the files (I guess not that bad a design for 25 years ago
with the tight memory constraints), but this paves the way for
supporting sprites in Vulkan.
This cleans up texture_t and possibly even improves locality of
reference when running through texture chains (not profiled, and not
actually the goal).
