This commit adds a node to distribute points inside of volume grids.
The "Random" mode usese OpenVDB's "point scatter" implementation, and
there is also a "Grid" mode for uniform distributions. Both methods
operate on all of the float grids in the volume, using every voxel with
a value higher than the threshold. The random method is not stable as
the input volume deforms.
Based on a patch by Angus Stanton (@abstanton), which was based on a
patch by Kenzie (@kenziemac130).
Differential Revision: https://developer.blender.org/D15375
b6e26a410c