I noticed that our slerp function sometimes produces a jolt in
animation, but only when the time delta is very small, and only when the
two operands have completely opposing signs.
For example, let's say you are slerping from <0.76, 0.39, 0.51, 0.19>
to <-0.72, -0.45, -0.49, -0.17>.
These quats are actually quite near to each other because the total
negation of the second quat is similar to the first quat.
We were already doing the short path check in the proper slerp path, but
not when falling back to lerp due to a small angle.
These constants are not part of the standard. We instead use our own
constexpr definitions in the filament::math namespace, as part of
the scalar.h include.
