[GRASS5] PNG and CELL driver aspect ratio

[Markus Neteler]

Hi again,

in general it would be great (probably not possible) to have
some magic inside d.mon/PNG/CELL to optionally adjust it
to the current region settings.
The idea is to avoid the black border if the GRASS_HEIGHT
GRASS_WIDTH settings do not match the current map extent.

But I have no idea how to do that. Maybe a flag is possible
to set GRASS_HEIGHT, GRASS_WIDTH according to rows and
cols of the current region. Of course I can do that manually,
but this looks like a nice job for a computer.

Just an idea for user's convenience,

 Markus