Fix several bugs in the animation system

Make the identity fallback matrix static so the pointers in interpolations
stay valid.

Don't crash when getting pose matrices from the first keyframe.

Matrix multiplication order for base point shift compensation was wrong.

Re-compensate for base point shift after interpolation.

Use correct comparison operator for max_index in AnimationPlayer.

Pass collection to keyframe loader to make loading poses possible.

Use Pose::set_armature rather than loading the pointer directly to get the
matrix array resized.