Do a check for valid num_files before doing
assert(!m_mesh_update_thread.IsRunning());

In particular, don't execute the assertion if all media has already
been received.

Also write something about rebuilding shaders to infostream in
Client::afterContentReceived()

Fixes a denial of service vulnerability, issue #784

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