A grid of createPbrMaterial() spheres: roughness goes
from 0.05 to 1 left to right, metalness from 0 (front row) to 1 (back
row). The floor has a generated sRGB base colour map and a normal map
applied without tangents, through screen-space derivatives. A
directional light circles like a low sun and a point light orbits with
an unlit marker as its child.