Blender Git Loki

Fix T47902: Particle grid not using modifier stack.

Disclaimer: this is tentative fix, seems to be working but you never know with particles.
If issue arise, we'll just revert this commit and hide 'use modifier stack' option
for grid distribution instead.

This commit also try to make choice of which dm to use during distribution more generic
(deduplication of code), again hopping not to break anything. :P