replace fontrenderer.h include with forward declaration

in order to flatten the build dependency tree further, include
fontrenderer.h directly where needed, in header files a forward
declaration is enough


Origin commit data
------------------
Branch: ni/coolstream
Commit: 2303d94300
Author: Stefan Seyfried <seife@tuxbox-git.slipkontur.de>
Date: 2017-02-05 (Sun, 05 Feb 2017)



------------------
This commit was generated by Migit

src/gui/lua/luainstance_helpers.h

@@ -46,6 +46,8 @@
 #define lua_unboxpointer(L, i) \
 	(*(void **)(lua_touserdata(L, i)))
 
+class Font;
+
 typedef std::pair<lua_Integer, Font*> fontmap_pair_t;
 typedef std::map<lua_Integer, Font*> fontmap_t;
 typedef fontmap_t::iterator fontmap_iterator_t;