take FOV into account when determining screen size of planets/stars
directly use the explicit vector3d->vector3f conversion
add SystemBody::GetAlbedo() to get a general/average color for a body

Not yet implemented in any sensible way. This just returns a constant
colour for all bodies.
use body 'albedo' colour rather than atmosphere colour for its billboard
reduce system-body billboard size threshold to 8 pixels (diameter)
Merge remote-tracking branch 'jpab/issue-2791'