- Cleaned up and refactored the AzimuthalProjection and GnomonicProjection.

- Turned GnomonicScanlineTextureMapper into a GenericScanlineTextureMapper.
- Got rid of all remaining projection specific code that used viewport->projection.
- Moved the Gnomonic projection to the right toolbutton menu.
- Prepared for further inclusion of "spherical" projections.
- Fixed a crash bug in the GnomonicProjection that would result from another
  division by zero.