Add an alpha channel to the brown ground texture

This channel is fully opaque, but its presence ensures the ground will be
drawn in the transparent pass, which is necessary for the mip fade to work.
Removing alpha test functionality for now

In the future, this can probably be handled by the shader on a case-by-case basis.
Heavy changes to the texture object feature set

Removed a number of facilities that were completely unused: subsampling and
support for 16-bit textures. Added configurable color scaling on a per-mipmap
basis.
Add mipmap scaling of ground alpha to give fade-out in the distance