Fix scene midpoint detection.
MaterialPreviewRenderer: always set depthmask=true, this fixes a visibility problem with some transparent materials.
Further simplify code to compute scene bounds.
Add ICameraController.SetPivot() and implement it for OrbitCameraController s.t. the scene orbits around the geometric center of the scene.
Merge branch 'master' of github.com:acgessler/open3mod
OrbitCameraController: SetPivot() now uses dirty flag.