Fix #10660: Sprite Font scale affected by viewport zoom level limits. (#10668)

(cherry picked from commit db573c8742)

# Conflicts:
#	src/fontcache/spritefontcache.cpp
#	src/gfx.cpp
#	src/spritecache.cpp

src/zoom_type.h

@@ -57,6 +57,7 @@ extern int _gui_scale;
 extern int _gui_scale_cfg;
 
 extern ZoomLevel _gui_zoom;
+extern ZoomLevel _font_zoom;
 #define ZOOM_LVL_GUI (_gui_zoom)
 
 static const int MIN_INTERFACE_SCALE = 100;