replace slotmap dependency with reduced local version

[cyrgani]

This showed a slight improvement in compile time for me, as slotmap is only used in texture.rs and most of it is not needed and many things can be simplified based on the limited use case.

[not-fl3]

Thanks, great work! Yeah, the whole slotmap deal was a big mistake, but with your fix we at least will not pay for it with compile time.