Initial support for using cyberarm_engine as a mruby mrbgem: Remove/disable usages of defined?, update old gosu mouse and keyboard constants, and replace one usage of window.button_down? with proper Gosu.button_down?

lib/cyberarm_engine/text.rb

@@ -149,6 +149,8 @@ module CyberarmEngine
     end
 
     def markup_width(text = @text)
+      text = text.to_s
+
       spacing = 0
       spacing += @border_size if @border
       spacing += @shadow_size if @shadow