Blender Git Loki

Optimization and threading fix for shapekeys weights calculation

This commit fixes two different issues, which were caused by
how weights are being calculated for relative shapekeys.

Weights for key block used to saved in KeyBlock DNA structure,
which lead to situations when different objects could start
writing to the same weights array if they're sharing the same
key datablock.

Solved this in a way so weights are never stored in KeyBlock
and being passed to shapekeys routines as an array of pointers.
This way weights are still computed run-time (meaning they're
calculated before shapekey evaluation and freed afterwards).

This required some changes to GameEngine as well, to make it
never cache weights in the key blocks.

Another aspect of this commit makes it so weight for a given
vertex group is only computed once. So if multiple key blocks
are using the same influence vertex group, they'll share the
same exact weights array. This gave around 1.7x speedup in
test chinchilla file which is close enough to if we've been
caching weights permanently in DNA (test machine is dual-code
4 threads laptop).

Some further speed is optimization possible, but it could be
done later as well.

Thanks Brecht for idea of how the things might be solved in
really clear way.