Fix 100% spawns not spawning with low spawn density

[Hirmuolio]

Summary

SUMMARY: Bugfixes "Make 100% spawns always spawn even with low spawn density"

Purpose of change

Fixes #36422

Describe the solution

The reduced spawn chance from low spawn density was applied too early resulting in 100% spawns not always spawning.

The reduced spawn chance is now checked only if unmodified spawn chance <100.

I also slightly changed how max_odds math is written. The math is same but. It is just is easier to read.

I also changed how spawning from group is handled. Instead of always spawning exactly one monster it will now spawn spawn_count * pack_size.get() monsters (in current use this is always one so this doesn't really change anything. Just makes behavior more consistent).

Describe alternatives you've considered

Testing

Set spawn density to 0.1 and the warehouse boss did spawn.

Additional context

This will probably affect other spawns too in other places where low spawn density caused no monsters to spawn sometimes. Based on code the fixed behavior is the intended way to work.

[Kodiologist]

Noice!