Easing and correct scaling for .anms, custom pivot points

[damoldavskiy]

The default blending code just multiplies the entire .anm transform matrix by blending * power, and sets the diagonal to one. This has several issues:

1. The transform becomes non-orthonormal, because rotation vectors are not normalized. That means, with some .anms the weapon is going to stretch in a weird way
2. Practically the rotation amount is limited. For example, .anm can't rotate the weapon 90 degree to the side
3. The blending is applied to rotation vectors directly, meaning the rotation speed isn't linear
4. The blending itself is linear parameter so it doesn't feel natural

To fix those, I convert rotation transform to a quaternion and apply slerp to ensure constant spherical speed of blended rotation (while translation is applied as before). Also, an interpolation func is applied to blend amount to make large-amplitude movements more or less natural looking.

Also added support for playing anms around custom pivot points.

Fixed blend_move_anims command to affect weapons with GLs as well.