change font scale based on ui scale, fix brush outline color

src/app.h

@@ -258,6 +258,7 @@ public:
     void ui_restore();
     void set_ui_rtl(bool rtl);
     bool get_ui_rtl() const;
+    void set_ui_scale(float scale);
 
     void cmd_convert(std::string pano_path, std::string out_path);