Symbols in form of PNG images enable to choose easily symbols in d.vect
and in g.gui.psmap dialogs. It is necessary to keep images synchronized
with symbols from ./lib/symbol/symbol/ directories.

How to create new symbol image
------------------------------

You can use following script (see its code for runtime requirements),
run script in symbol group directory (e.g. ./lib/symbol/symbol/basic)
within a GRASS session. ImageMagic should take care of compressing the
image as much as possible.

    tools/symbol_to_img.sh

Image should have 30x30 px to be displayed correctly in GUI dialog. If the symbol
has different width and height, you are supposed to correct it (for example
in Gimp see Image -> Canvas size). Also consider where the reference point of symbol
is placed (see e.g. offbox_ne, offbox_ns symbols). If you don't do that
it will be badly displayed in the GUI.
