This avoids an issue where the skins array takes up a fixed, but large
amount of memory at runtime. On x86_64 linux, that array is roughly 80
megabytes in memory, most of which is unused when the game is unmodded.
Instead, we treat `skins` as a dynamically resizing array, and it is an
array-of-pointers into separate allocated `skin_t`.
This is based on Lactozilla's skin limit MR for SRB2, but I've rewritten
it because RR has diverged quite a bit.
This was verified to check every access of `skins` by using clangd's
find-all-references function. However, I have only tested plain skins,
not Lua addons, so that could afford some extra checking.