[GRASS5] Transparency added

[Huidae Cho]

Oops! the attached file is too big.  Try this:
http://geni.ath.cx/grass/transparency.png

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I've added transparency feature to display drivers (XDRIVER and PNG) and d.rast
and d.vect now have transparency= (%) option.  You can find a screenshot
attached in which two rasters and one vector are overlaid.

# opaque drawing
d.rast dem
# 80% transparency, -o is needed not to clip previous drawings
d.rast landuse trans=80 -o
# 90% transparency with blue area fill
d.vect subbasins trans=90 fcolor=blue

Please find attached the png file.

I hope you enjoy this.
Huidae Cho