Use class SkyMap as the paint device instead of class KStars in
Legend. This is because the --dump command line option does not
instantiate the KStars class, but instantiates SkyMap and
KStarsData.

For the same reason, also obtain the KStarsData instance using
KStarsData::Instance() rather than KStars::data()

(The crash on --dump is not yet fixed due to other problems. This is
the first step towards the fix)

No significant regressions were seen in the dumped sky images.