improve stack management
improve stack management

- use new spend member to track end of stack
- in Actionscript, the stack is infinite (initially all values are undefined)
  Implement this concept via swfdec_script_ensure_stack
- don't pass the number of free stack spaces to functions anymore now that
  we know where the stack ends

mltframework/swfdec,freedesktop-unofficial-mirror/swfdec__swfdec,freedesktop-unofficial-mirror/swfdec__swfdec,freedesktop-unofficial-mirror/swfdec__swfdec,mltframework/swfdec