ssreflections: if we're going to (pretend to) take 4 diffuse samples, then...

ssreflections: if we're going to (pretend to) take 4 diffuse samples, then take them in a diamond pattern instead of all in a line.  this also slightly simplifies(?) the shader.
(transplanted from 7b334c22ece4e98565c5d182690ab9ca4c2526c6)