function pixelAngle
lens.ts:187
The angle one pixel subtends at the center of the view, radians.
The reciprocal of pixelsPerRadian, and stated as its own function because
the two are the identity the terrain predicate and the LOD thresholds both
stand on — a body a third of a pixel across and a patch cell sixteen pixels
across are the same claim measured in opposite directions.
Returnsnumber