Check case-sensitivity (incl. in `gl.Texture`)

See https://github.com/beyond-all-reason/spring/issues/1180#issuecomment-1890384415

> If you load a texture with a different capitalization, for example:
gl.Texture("mytexture.dds")
gl.Texture("MYTexture.dds")
Each different capitalization will load the same file into different texture objects, and not used the same cached texture object.

The above is a specific example of incorrect case-sensitivity. The second part of the task is to figure out whether there are more.

Start with the linked issue comment and the gl.Texture entry point, then trace texture loading and cache-key handling. No files or tests are named in the issue; done means equivalent path capitalization reuses one texture object and other case-sensitivity problems are identified and addressed.

Written by the indexing model from the issue text.