Where appropriate, update references to geomview license too,
now called COPYING.geomview.

Once we pick a license this might change, but
in the mean time, at least the NCSA UIUC origin is noted.

from previous time.  Add TXF_OVER txflags bit.

Encode blending-status bits in along with the opengl texture id.

get_dsp_context ad MAXDSPCTX.

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).