Fix special text box images sometimes flashing

These special images are the crewmates, Level Complete, and Game
Complete images. They flashed depending on if you were lucky and
happened to got your delta-timesteps just right when text boxes were
fading in and out.

Honestly, I'm surprised text box fading in/out hasn't ran into this
issue before. It's insane luck that this issue hasn't occurred before or
anything.

Well, anyways, to fix this, there's now an attribute `allowspecial` on
text boxes, and an optional parameter of the same name for
Graphics::createtextbox(). This attribute is the only thing that will
let these special text box images render. And any createtextbox()es that
utilize these special images have been updated accordingly.

desktop_version/src/Textbox.h

@@ -45,6 +45,8 @@ public:
 
     int max;
 
+    bool allowspecial;
+
 };
 
 #endif /* TEXTBOX_H */