Make sure sprite colors in the editor don't update more than 30 FPS

This adds Graphics::crewcolourreal(), which is like the
entityclass::crewcolour() that the editor already uses, except for the
real color instead of the color ID. Also, editorclass now has an
attribute `entcolreal` so enemy colors don't update more than 30 frames
a second.

desktop_version/src/Graphics.h

@@ -322,6 +322,8 @@ public:
 	void updatetitlecolours();
 
 	bool kludgeswnlinewidth;
+
+	Uint32 crewcolourreal(int t);
 };
 
 extern Graphics graphics;