Post-multiplies the 4x4 transformation `matrix` (a 16-element flat
array of floats, as produced by id_matrix()) by a rotation of
`degrees` about the X axis, computing `matrix` = `matrix` * rotation.
The angle is given in DEGREES, not radians (it is converted to radians
internally).
The passed `matrix` array is modified IN PLACE: its 16 elements are
overwritten with the result, and that same array is left on the stack
as the return value. rotate_x() mutates its first argument; the return
value is the very array you passed in, not a copy.