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DeformMod: Performance by reusing buffers

The Deform modifiers was reallocating buffers that only fit the vertices
of the inner loop. This patch first counts the maximum needed buffer and
allocates one.

When using the daily dweebs animation file the playback performance went
from 0.66 fps to 0.93 fps.

Reviewed By: sybren

Differential Revision: https://developer.blender.org/D7132