Fixed|Client|World|Particle Generators: Stuck loading Heretic E1M8 with "Decor-decor.pk3" add-on

A missing spawn density scale factor resulted in the attempt to spawn
many magnitudes more particles than intended by the add-on author.

The constant 128x128 map units squared scaling factor was seemingly
inadvertently removed whilst refactoring.

Plus cleanup.
Merge branch 'master' of github.com:skyjake/Doomsday-Engine