BBBikeUtil: new function is_in_path_cached

With tests in t/bbbikeutil.t.
svg2photo: prefer rsvg-convert over convert (ImageMagick)

rsvg-convert seems to be much faster than convert (often below 0.1s on
my FreeBSD system, while ImageMagick needs more than 1s). Anyway
ImageMagick delegates svg rendering to another program. On my system
it's delegated to inkscape.