Fix for Sintel hair bug.

The hair solver needs sane input to converge within reasonable time
steps. In particular the spring lengths must not be too difference
(factor 0.01..100 or so max, this is comparable to rigid body simulation
of vastly different masses, which is also unstable).

The basic hair system generate strands with equally spaced points, which
is good solver material. However, the hair edit operators, specifically
the cutting tool, can move points along the strands, creating tightly
packed hair points. This puts the solver under enormous stress and
causes the "explosions" observed already during the Sintel project.

The simple solution for now is to exclude very short hairs from the
simulation. Later the cutting tool should be modified such that it
keeps the segments roughly at the same length and throws away vertices
when the hair gets too short (same goes for the extension tool).

The hair system should have a general mechanism for making sure that
situations such as this don't occur. This will have to be a design
consideration for replacements in any future hair system.

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