Merge branch 'develop' into issue366
mapImage() avoid drawing off-page polygons

Speeds up test 04.R by 33 percent, when the graph is set to show world
topography in domain 20S to 20N 60W to 60E.  Since this is showing just
7 percent of the data, the speedup is not proportional to the clipped
fraction, probably because R is already good at clipping polygons.
Still, it doesn't use too much memory (6 logical values per polygon) so
the change may be worthwhile.
remove timing test
Merge branch 'issue366' into develop