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Fix T43782: smoke simulation problems with multiple touching colliders.

When the colliders are joined, each cell that touches them only gets
velocity contribution once. When there are multiple objects, velocities
are summed, which causes some cells to get 2x, 3x or more than the actual
object velocity.

Fix this by using the average velocity of all colliders touching a cell.

Reviewers: miikah, lukastoenne

Reviewed By: lukastoenne

Subscribers: dafassi, scorpion81, #physics

Maniphest Tasks: T43782

Differential Revision: https://developer.blender.org/D2112