svg2png: Speed up gimp startup with -d option.

This option prevents gimp from loading patterns, gradients, palettes,
and brushes.  I think that these are mostly for interactive use.  I didn't
see any difference in output quality.

On my system this sped up "make -f Smake -j128 icons" from 1.8 to 1.6
seconds.  On other systems I guess it will make a bigger difference.