Mathutils: add a Matrix.LocRotScale constructor for combining channels.

Combining location, rotation and scale channels into a matrix is
a standard task, so while it is easily accomplished by constructing
and multiplying 3 matrices, having a standard utility allows for
more clear code.

The new constructor builds a 4x4 matrix from separate location,
rotation and scale values. Rotation can be represented as a 3x3
Matrix, Quaternion or Euler value, while the other two inputs
are vectors. Unneeded inputs can be replaced with None.

Differential Revision: https://developer.blender.org/D11264

doc/python_api/examples/mathutils.Matrix.py

@@ -14,10 +14,14 @@ mat_rot = mathutils.Matrix.Rotation(math.radians(45.0), 4, 'X')
 mat_out = mat_loc @ mat_rot @ mat_sca
 print(mat_out)
 
-# extract components back out of the matrix
+# extract components back out of the matrix as two vectors and a quaternion
 loc, rot, sca = mat_out.decompose()
 print(loc, rot, sca)
 
+# recombine extracted components
+mat_out2 = mathutils.Matrix.LocRotScale(loc, rot, sca)
+print(mat_out2)
+
 # it can also be useful to access components of a matrix directly
 mat = mathutils.Matrix()
 mat[0][0], mat[1][0], mat[2][0] = 0.0, 1.0, 2.0