* The old collisions code detected particle collisions by calculating the
collision times analytically from the collision mesh faces. This was
pretty accurate, but didn't support rotating/deforming faces at all, as
the equations for these quickly become quite nasty.
* The new code uses a simple "distance to plane/edge/vert" function and
iterates this with the Newton-Rhapson method to find the closest particle
distance during a simulation step.
* The advantage in this is that the collision object can now move, rotate,
scale or even deform freely and collisions are still detected reliably.
* For some extreme movements the calculation errors could stack up so much
that the detection fails, but this can be easily fixed by increasing the
particle size or simulation substeps.
* As a side note the algorithm doesn't really do point particles anymore,
but uses a very small radius as the particle size when "size deflect" isn't
selected.
* I've also updated the collision response code a bit, so now the particles
shouldn't leak even from tight corners.
All in all the collisions code is now much cleaner and more robust than before!
Janne Karhu
60ce95f562