Create a new {@link Matrix2d} and make it a copy of the upper left 2x2 of the given {@link Matrix3d}.
@param mat the {@link Matrix3d} to copy the values from
See Implementation
Create a new {@link Matrix2d} and make it a copy of the upper left 2x2 of the given {@link Matrix3d}.
@param mat the {@link Matrix3d} to copy the values from