Supernova sun glare and fade regressions

[Goober5000]

There are a few regression bugs that were introduced at some point -- including but possibly not limited to ffc14f5 -- related to how the sun glare is treated during the supernova impact.

The attached image sequence shows what it looks like in retail...

1. When the supernova is close, the glare should affect the whole screen even when the player is not looking at the sun. This does not currently happen in FSO. The Sun_spot variable is supposed to color the screen using the same mechanism as the red "pain flash".
2. The glare is a lot more dramatic in the retail letterbox scene than the FSO letterbox scene.
3. In retail, the glare always affects the camera's view of the player. In FSO, this does not happen unless the sun happens to be near the camera's view direction.
4. The screen fades to white in retail. In FSO this is again dependent on the camera direction.

Part of this is due to the introduction of light shafts in the above referenced commit. When I modified gr_lightshafts_enabled() to return false if a supernova was in progress, this almost entirely fixed the problem - the glare behaved as it was supposed to. However there are still two deficiencies:

1. Disabling light shafts when a supernova starts could abruptly remove any lightshafts currently on the screen.
2. During the letterbox scene, the entire screen faded to white, not just the part inside the letterbox.

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[Goober5000]

With the merge of #4153 the glare now behaves properly, but the two deficiencies mentioned in the top post still need to be resolved.

[EatThePath]

I have an idea or two to try, no promises they'll pan out

[EatThePath]

Okay my attempt didn't work.
should the glare be visible when you're looking directly away from the star, or just when it's only slightly off screen?

[Goober5000]

The glare should tint the screen even if the player is facing directly away from the sun. See the first screenshot from retail.