Draw the FS1 nebula over a skybox model when a mission asks for it

[dustinspace217]

What?

Adds an optional mission line, $FS1 Nebula Over Skybox: YES, at the end of the background section, for the FS1 nebula introduced in #7853. When it is set and the mission has a skybox model, the FS1 nebula is drawn over the skybox with additive blending. qtFRED and FRED get a "Draw over skybox" checkbox for it. Without the line, a skybox still hides the nebula, as in #7853.

Why?

#7853 skips the FS1 nebula whenever a mission has a skybox model, because the mesh is opaque and would hide the skybox. That turns out to cover the missions most likely to want the FS1 look: every FSPort mission and every retail mission as rewritten by MediaVPs 5.x carries $Skybox Model: starfield.pof (the model ships with the MediaVPs, so with them the skybox loads and the nebula is hidden). Mod makers should not have to give up their skybox to get the nebula. Additive blending keeps the skybox and its stars where the nebula is dark and adds the nebula's colour where it is bright.

How?

The parser reads the line at the end of the background section (after $Environment Map:) into a per-mission flag, reset with the other nebula state, in the engine and in the Lab. stars_draw lets the nebula through when the flag is set even with a skybox model, and nebula_render switches the material to additive blending in that case; the no-skybox path is untouched. The environment map follows the same rule, so reflections show what the player sees. The saver writes the line only when the flag is set and a pattern is selected, with a version comment; the position means builds that do not know the line skip it with the rest of the section. qtFRED's background editor model gets a getter and setter and the dialog a checkbox wired like the other old-nebula controls; FRED's background dialog gets the matching checkbox with DDX, enabled with the colour combo and disabled under full nebula.

Screenshots?

Screenshots of FSPort's SM1-01 (with the FSPort MediaVPs) with and without the line, and of MediaVPs 5.0.2's SM1-01 with the line, follow in a comment.

Testing?

Linux, headless harness, OpenGL and Vulkan: FSPort's SM1-01 with the FSPort MediaVPs (so its skybox loads) draws the nebula over the skybox with the line and nothing without it, same as with the switch off; MediaVPs 5.0.2's SM1-01 with the line draws it over the purple sky; retail SM1-01 (no skybox) with the line is pixel-identical to #7853; a build without this change reads the mission with the line and renders it exactly as without the line, backgrounds intact. qtFRED built from the branch saves the line at the end of the background section; the checkbox itself could not be driven by the harness, so the editor-set save path is verified by reading. FRED has eight changed lines, syntax-checked against the engine headers, not built here; the MSVC CI jobs are its compile check. In standard-format saves the line is written plain; older builds skip it and show no nebula over the skybox.

Anything Else?

Depends on #7853; opened as a draft until that lands. I used Claude Code to write and test the change; the design and the testing calls are mine.

[dustinspace217]

Screenshots from the headless harness (OpenGL, 1920x1080). All three missions have $Skybox Model: starfield.pof and the FS1 nebula fields; the only difference between the first two is the new line.

FSPort SM1-01 with the FSPort MediaVPs, line absent (same frame as #7853 and as the switch off):

Same mission with $FS1 Nebula Over Skybox: YES (Nebula03, Gold, pitch 90):

MediaVPs 5.0.2's SM1-01 with the line (Nebula01, Red, pitch 90), drawn over its own sky art:

[dustinspace217]

Superseded by #7853, which now draws the nebula over a skybox model as part of the merged design.