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/spritecache.h

@@ -61,6 +61,7 @@ static inline const byte *GetNonSprite(SpriteID sprite, SpriteType type)
 
 void GfxInitSpriteMem();
 void GfxClearSpriteCache();
+void GfxClearFontSpriteCache();
 void IncreaseSpriteLRU();
 
 SpriteFile &OpenCachedSpriteFile(const std::string &filename, Subdirectory subdir, bool palette_remap);