Replaces the 4x4 transformation `matrix` (a 16-element flat array of
floats, as produced by id_matrix()) with a viewing-rotation matrix
that orients toward the look point (`look_x`, `look_y`, `look_z`).
Unlike translate()/scale()/rotate_*(), this efun does NOT
post-multiply; it derives a fresh matrix from the current one:
- The eye position is taken from the current matrix's translation
column (elements 12, 13 and 14).
- The current matrix's first column (elements 0, 4 and 8) is used as
the up reference.
- The forward, right and up axes are recomputed and normalized to
look from the eye toward the look point.
The passed `matrix` array is modified IN PLACE: its 16 elements are
overwritten with the computed matrix, and that same array is left on
the stack as the return value. lookat_rotate() mutates its first
argument; the return value is the very array you passed in, not a copy.
Because the eye and up reference come from the matrix itself, populate
the matrix with the eye position (for example via id_matrix() then
translate()) before calling this efun. Use lookat_rotate2() to supply
both the eye point and the look point explicitly instead.