TextureSource has a list of textures to delete (m_texture_trash) so this
provides a proper, non-hacky way to delete RTT textures. Also, the prior,
hacky way of deleting them seems to be broken (see pull request #803).

To avoid header file clutter by repeating the same long list of
arguments over and over again, store the arguments of
generateTextureFromMesh in a struct called TextureFromMeshParams.

Also fix issue #782 (Only use bilinear (and others) on item textures
when settings allow it).

Cavegen: Prevent caves from occuring above ground level, and superfluous mixing of lava and water in caves

LuaVoxelManip: Fix minor bug with set_lighting, remove coordinate params for light and liquid updates

Two new configuration options are added:
     - "enable_ipv6" to enable/disable the overall use of IPv6
     - "ipv6_server" to enable/disable the use of IPv6 sockets when running
       a server (when "enable_ipv6" is enabled)