    Normally there are an infinite number of repeating bump_maps created over
 every unit square of the X-Y plane like tiles.  By adding the keyword 'once'
 after a file name, you can eliminate all other copies of the bump_map except
 the one at (0,0) to (1,1).  Areas outside this unit square are treated as
 fully transparent.

    NOTE: The "once" keyword may also be used with image_map and material_map
 statements.
