[GRASS5] Driver updates

[Glynn Clements]

I've made a couple of changes to XDRIVER and the PNG driver:

1. XDRIVER now uses Pixmaps to store panels, rather than files. The
files are still created, but they only store the Pixmap XID and the
panels' bounding box; the actual data is stored in the Pixmap. This
should make code which uses panels usable where the connection to the
X server is slow (as the image data doesn't travel over the X
connection).

2. Two new features have been added to the PNG driver. First, it can
now write PPM files as well as PNG files; if you set GRASS_PNGFILE to
a filename which ends in ".ppm", a PPM file will be created. Second,
if the environment variable GRASS_PNG_AUTO_WRITE is set to "TRUE", the
image file will be written after each operation (i.e. whenever a
client disconnects), rather than only being written out when the
driver terminates.

The PPM support might be useful for Tcl/Tk applications; PPM is one of
the formats which Tk's "image create photo ..." command understands.

-- 
Glynn Clements <glynn.clements at virgin.net>