Fixes two issues in camera navigation:
- Unintuitive camera rotation
- Wrong center point when zooming in/out and rotating
See https://github.com/qgis/QGIS-Enhancement-Proposals/issues/215
This PR employs the following changes:
- The zoom in functionality will zoom in towards the real 3D position of an object in the scene instead the camera view center point used previously.
- The rotation will use the real clicked 3D position of a pixel as well instead of the camera view center point.
- The press and drag behaviour is improved to shift the map in real 3D coordinates instead of some arbitrary measurement (you can see the clicked pixel following the cursor instead of drifting away).
