add Font::get_advance()

src/gui/Font.h

@@ -10,6 +10,7 @@ class Font
 public:
     bool load(const char * fname, int size);
     std::shared_ptr<Glyph> get_glyph(FT_ULong character);
+    int get_advance() { return m_advance; }
 
 protected:
     bool preload_glyphs();