Xonotic release builds fail to start when user has no config.cfg

This happens with release and autobuild versions using DDS compressed textures.
When menu.dat detects a build using DDS textures, it checks if gl_texturecompression is enabled and if it's not, sets gl_texturecompression 1 and calls r_restart, triggering the bug:

Engine Error: Mem_Alloc: pool == NULL (alloc at ../../../model_shared.c:4481)

This breaks the game for new users unless they manually add gl_texturecompression 1 to config.cfg.

Bug history:
In it's current form the bug was introduced by 4d33dcb2 which was a fix for

Host_Error: R_FreeTexturePool: pool already freed
Engine Error: Host_Error: R_FreeTexturePool: pool already freed

and that bug was introduced by ca03733f

Mod_Mesh_Destroy() was added in 2bac6070 but it was never called anywhere until ca03733f and thus was untested until that point.