terrain: Fix glx-viewport-draw method.

terrain: Fix glx-viewport-draw method.

Remove redundant (now incorrect) call to with-glx-widget (i.e.
glXMakeCurrent).  GL no longer allows recursive locking.

Fix %trace to use outf-error instead of generic IO.  The latter may
suspend-current-thread during a callback.

Punt glColorMaterial and much frobbage (e.g. setting params to their
defaults) copied from examples.  Minimize differences between
draw-colored and draw-shaded.