Shadow isn't rendered properly on certain angles

On DSDR 4.1 with this camera state:

```
{
px = 3240.77271
py = 1105.42786
pz = 5310.42676
dx = -0.6401963
dy = -0.7544509
name = rot
fov = 45
mode = 3
dz = -0.1447501
oldHeight = 561.879944
},
```

I get this on top left corner:

If I move the angle just a bit it renders properly:

Reproduce the shadow artifact using the supplied DSDR 4.1 camera state, then compare it with the nearby angle that renders correctly. Trace the shadow-rendering entry point and inspect the angle-dependent path; done means the shadow renders correctly at the reported angle as well as the adjacent angle.

Written by the indexing model from the issue text.