Blender Git Loki

Enhance vgroup handling when merging meshes.

We were looping over all vgroups in destination mesh and making string
comparison, for every vgroup of every vertex of merged mesh! Crazy!

Now we simply create a temp mapping of vgroup indices, seriously
simplifies things (and gives significant speedup when merging huge meshes
with lots of vgroups, here with quick stupid test went from 120ms in
vgroup merging to less than 5ms, 25 times quicker!).