Bug: $Radar Image Size affects colors of the radar icon and can make the icon invisible

When $Radar Image Size is set at or below half the larger dimension of the image, the radar icon is invisible. When set just above half the larger dimension, the icon is much less visible (I can't tell which).

Here are some [screenies](https://imgur.com/a/I18fsd0) with a 32x8 icon, becoming invisible at $Radar Image Size: 16. I replicated the behaviour with a 256x64 icon, showing the same behaviour with total invisibility starting at $Radar Image Size: 128.

I see this behaviour in FSO 19 and the Dec11 nightly.