Displacement shaders: moving surface points

After a surface has been tessellated, its surface points can be moved by a displacement shader.

Displacement along normal, not along normal.

TO DO: figure of 5x4 pt grid similar to figure 8.3 of ARM book.

Recalc normals.

RixDisp() ?

Talk about common trick for adding back smoothness?  Also, for some surface types a smooth shading normal will be automatically computed for each ray hit.

Displacement bounds

BVH bbox tightening.  Not too large: important for ttfp.  Not too small: holes.

TO DO: Figure with holes due to too small displacement bound.