Raise skin limit to 256 + Skin related code cleanup

Backport of STJr/SRB2!1466 (merged) with some additions

Removing WIP from the original merge request to this because i have made it functional.

This could be useful for servers with many characters in mind, and as skins are now allocated dynamically, it's not a waste of memory for servers that won't use all the character slots.

Here's a video recorded by @raphaelgoulart on a server with 211 characters: lotsofchars

Additions:

• This MR makes the full use of HWR_InitMD2, that previously was executed with no skins loaded.
• Removes message "Unknown sprite/player skin..." at startup with opengl because it doesn't make sense as you can use models with characters added after startup.
• Removes all code related with character descriptions (leftover from srb2).

Testing:

• 32 Bit Windows Build compiled with DEVELOP and ZDEBUG flags. srb2kart-raise-skin-limit.exe
• 32 Bit Windows Moe Mansion build. srb2kart-raise-skin-limit-moe.exe

Please let me now if anything is wrong.