[Bf-funboard] Adaptive Particle Counts

[Zauber Paracelsus]

This is a little idea I had rolling in my head, with regard to particles 
and hair:

- With Advanced particle settings enabled, a new option could be added 
to the Emission shelf (is that the right term?), under the Number 
entry.  The new option would be Adaptive Emission.

- When Adaptive Emission is enabled, it dynamically determines how many 
particles to emit, based on the Particles per Face setting.  As opposed 
to the current behavior, where it will try to fit in any leftover particles.

- Additionally, a Particles-to-Distance setting can be added.  It would 
override Particles/Face and instead make the system emit a particles 
spaced out by X blender units.

- The emission Number setting would still be used, and instead would 
serve as a maximum limit on the number of particles.

- Going into Particle Edit mode and making changes will, of course, 
prevent the adaptive emission from adding or removing any further particles


Advantages:

- Makes it so that the particles/face setting guarantees a specific 
number of particles (in case that is important to the artist)

- Makes particle systems more reusable by allowing them to adapt their 
particle counts to the emitter mesh, instead of creating multiple 
variants of a particle system for each mesh it gets used on.  Useful for 
when particles are used for "splatting" meshes onto a scene, such as 
trees, shrubs, rocks, or the buildings in a city (the latter of which 
I've done).