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.cpp

@@ -23,6 +23,8 @@ textboxclass::textboxclass(void)
     max = 0;
 
     flipme = false;
+
+    rand = 0;
 }
 
 void textboxclass::centerx(void)