Auto merge of #26352 - asajeffrey:webxr-glwindow-red-cyan, r=Manishearth

Add a pref for webxr glwindow anaglyphic red-cyan stereo

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Allows us to test out 3D content using old-skule glasses.

---
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- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes do not require tests because it's back end work

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components/config/prefs.rs

@@ -297,8 +297,12 @@ mod gen {
                     enabled: bool,
                     #[serde(default)]
                     test: bool,
-                    #[serde(default)]
-                    glwindow: bool,
+                    glwindow: {
+                        #[serde(default)]
+                        enabled: bool,
+                        #[serde(rename = "dom.webxr.glwindow.red-cyan")]
+                        red_cyan: bool,
+                    },
                     hands: {
                         #[serde(default)]
                         enabled: bool,