add Font::get_line_height()

src/gui/Font.h

@@ -11,6 +11,7 @@ public:
     bool load(const char * fname, int size);
     std::shared_ptr<Glyph> get_glyph(FT_ULong character);
     int get_advance() { return m_advance; }
+    int get_line_height() { return m_line_height; }
 
 protected:
     bool preload_glyphs();