lightmap allocation speed-up, add -fastallocate

lightmap allocation speed-up, add -fastallocate

- Allocate large lightmaps first to speed up the allocation process
- Add `-fastallocate` option to trade lightmap size against allocation time (useful with hi res lightmaps on large maps: reduce allocation time from days to minutes for only some extra bytes:
- If fast allocation, skip lightmap files that are more than 90% complete
- If fast allocation, do not test allocation on every pixels, especially for large lightmaps

Thanks to Kangz (Corentin Wallez) for his precious help and his kind availability.