Fix Volumetric Nebula Bugs and Add Smoothing

[BMagnu]

This fixes three bugs in volumetrics:

1. Sometimes, volumetric nebulae would suddenly darken a lot when viewed against the sun. This was due to numerical imprecision in emissive handling. Clamping the emissive inputs from 0 to 1 fixes this
2. Oversampling was calculating its indices and divisors incorrectly, causing two resulting symptoms. For one, the nebula's edge was less properly smoothed and more artificially noisy, causing a slightly blocky appearance. Additionally, it would cause the volumetrics UDF to be incorrect within the nebula. This isn't a huge issue in and of itself, but would cause problems if anyone tries to rely on it in the future.
3. It now properly alpha-blends nebula and background, instead of doing so purely additively.

Furthermore, if I'm already touching the oversampling logic, I can also add index-shift logic that allows for smoothing the volume texture rendered from the POF, which allows to have volumetric nebula with less hard edges.

To illustrate this, three screenshots.

1. Old / Broken Behaviour:

2. New Behaviour: 4. Smoothing set to 1/16th:

While I have added missionsave code for the smoothing property, I currently do not have a machine with VS installed or a compiler setup that would be able to build or test FRED, as such, the new property cannot yet be set through the GUI, but it should persist if added in a text editor.

[wookieejedi]

Very nice update and looks straight forward!