Lots more caching (e.g. of texture slot numbers and texture id's) for speed.

txbind(NULL, NULL) invalidates currently-bound-texture-id cache.
Allow texture index 0 to hold a texture.  This allows for
kludges like using a [0..1) coordinate as a constant texture index.

Handle new texture attributes: alpha (-> 2-channel tx with luminance=0),
intensity (luminance=1, alpha=brightness).