Blender Git Loki

BLI_math: add mat3_polar_decompose, interp_m3_m3m3 and interp_m4_m4m4.

mat3_polar_decompose gives the right polar decomposition of given matrix,
as a pair (U, P) of matrices.

interp_m3_m3m3 uses that polar decomposition to perform a correct matrix interpolation,
even with non-uniformly scaled ones (where blend_m3_m3m3 would fail).

interp_m4_m4m4 just adds translation interpolation to the _m3 variant.