Update Assimp support, and minor Makefile and back-end improvements

Bring up-to-date Assimp library support for model loading from a
large variety of file formats of varying complexity. The generic
model file load interface functions have also changed somewhat.
Whether the library has been compiled with Assimp support is, to
some extent, transparent (any native loader may be used if
Assimp is not present). The Assimp library is available in Debian.

Improve Makefile build dependencies.

Clean up the back-end interface somewhat by regularizing back-end
function names a few other minor changes.

Signed-off-by: Harm Hanemaaijer <fgenfb@yahoo.com>