Blender Git Loki

Cloth: implement support for a hydrostatic pressure gradient.

When a fluid is put under influence of gravity or acceleration, it
forms an internal pressure gradient, which causes observable effects
like buoyancy. Since now cloth has support for simulating pressure
changes caused by fluid compression or expansion, it makes sense to
also support the effects of gravity.

This is intended for better simulation of objects filled or
surrounded by fluids, especially when constrained by collisions
or pinned vertices, and should result in more realistic shapes.

Obviously, this doesn't actually simulate fluid dynamics; instead
it is assumed that the fluid immediately adapts to changes in the
shape or acceleration of the object without friction or turbulence,
and instantly reaches a new static equilibrium.

Differential Revision: https://developer.blender.org/D6442