Tweak debug code and make some refactoring.

- Initialize rt.ray.Ray.flags in debug mode with RayFlags.Debug, instead of manually setting it.
- Expose rt.renderer.renderPixelNoAA(..) method (make it public).
- Use renderer.renderPixelNoAA(..) in gui.rtdemo.RTDemo.printMouse(),
instead of camera.getScreenRay(..) + renderer.raytrace(..).
- If prepass is not enabled do not do it, even if GI is enabled in rt.renderer.renderRT().
- Remove unnecessary bucket box clipping in rt.renderer.drawRect(..).
- Call printDiagnostics() only in debug mode in app.d.
- Remove unused Raytrace(..) method in rt.scene.d.