Don't update crewmate colors in text boxes every deltaframe

Colors in over-30-FPS mode shouldn't be updating every deltaframe;
mostly to ensure determinism between switching 30-mode and over-30 mode.
I'm going to overhaul RNG in 2.4 anyway, but right now I'm going to fix
this because I missed it.

The RNG of each special text box is stored in a temporary variable on
the text box itself, and only updated if the color uses it (hence the
big if-statement). Lots of code duplication, but this is acceptable for
now.

desktop_version/src/Textbox.h

@@ -45,6 +45,8 @@ public:
 
     /* Whether to flip text box y-position in Flip Mode. */
     bool flipme;
+
+    int rand;
 };
 
 #endif /* TEXTBOX_H */