v1.5.1 release candidate

- multiple lightgrid brushes now affects the mapping of lightgrid. Grid
points outside of brushes are now skipped by tracer (flooded with
nearest points).
- dark mode eliminated (it was ugly after all)
- better tracing of triangle models (removing inhibitRadius from all
traces except for lightGrid)
- lightGrid now can be multisampled (-gridsuper 2 gives 8 samples per
grid point, -gridsuper 3 gives 64 samples)
- lightGrid is now traces as blocks, with a number of visible
lightsources calculated for each block (an optimization).
- new commandline arguments for LIGHT: -ambient (mimicks _ambient),
-minlight, -minvertex, -mingrid, -gridsize "x y z"
- cheap mode removed