Blender Git Loki

Fix T38928 Particle add brush would generate trash on meshes with use_modifier_stack enabled

Issue here is complex (Of course, this is particles!)

First issue is that use_modifier_stack will use the num parameter of the
particles instead of num_dmcache, something the brush code did not
account for at all. Now correctly set DMCACHE_ISCHILD in that case.

Second issue is that make_derived_deform will return a mesh with less
indices than the particle system derived mesh. This would mean that
subsequent sampling of the particle derived mesh to initialize the
particles woould also produce garbage. This was being done for
optimization but in that case it broke the system.

Reviewers: lukastoenne

Differential Revision: https://developer.blender.org/D429