Fix planar projection focal calculation

Author: thatcomputerguy0101

src/projection.rs

@@ -336,7 +336,7 @@ impl<S: BaseFloat> From<PlanarFov<S>> for Matrix4<S> {
  }
 
  let two: S = cast(2).unwrap();
- let inv_f = Rad::tan(persp.fovy / two);
+ let inv_f = Rad::tan(persp.fovy / two) * two / persp.height;
 
  let focal_point = -inv_f.recip();