-------------------------------------------
var,funct (left-click): write TIFF (or RGB) file
  set rgb <outfile>
  save_rgb
-------------------------------------------

Write out current view in GL window as a tiff
file (or an SGI rgb file, if $tiffoutflag if
first set to 0) using name in entry to left
(after expansion if abbreviated).

Interface click must confirm overwrite (save_rgb
doesn't ask).  Mouse pointer is ignored.  Window
is popped to front and redrawn before saving.
Saves cursor.

The unlabeled button to the left of "W" button
doesn't pop/redraw window first, so tmp overlays
(e.g., time courses) are also saved into bitmap.
