Take field of view into account when calculating LOD

Does #2077.

Here is an example at three different resolutions & FOVs, taken just when a LOD defined at 100px is selected. You'll see that the monkey head is the same size in each, and if you measure the red or blue axis it's more or less 100 pixels (inaccuracy mostly from zoom keys...)

http://luomu.github.io/pioneer/issues/2077-fov/800_fov45.jpg
http://luomu.github.io/pioneer/issues/2077-fov/1024_fov85.jpg
http://luomu.github.io/pioneer/issues/2077-fov/1280_fov90.jpg