* Implemented supports for timeline marks in scene_producer. A mark is set on a speci...

* Implemented supports for timeline marks in scene_producer. A mark is set on a specific frame and can be of type start, stop, jump_to and remove:
  - A start mark has a name and can be started via call() with "play()" as first parameter and the mark label as the second. A jump_to mark will also make the given start mark be played from (makes looping animations possible). Going to a start mark always causes the scene to unpause.
  - A stop mark on a frame makes the scene pause until a call("play()", mark_label) occurs.
  - When the timeline reaches a jump_to frame the corresponding start mark is seeked to.
  - A remove mark removes the entire scene (saving resources after outro).
  Special start marks are "intro" which will be the first frame if not defined, and "outro" which will be an empty frame if not defined.
  call() with "next()" as first parameter will jump to the next start mark from the current position.
* Created cg_proxy for scene producer going to play mark "intro" on play() and "outro" on stop(). next() jumps to the next start mark from the current frame.
* Implemented interlaced support in scene_producer.
* Implemented support in psd_producer to use the timeline mark features in scene_producer. The marks are stored in the comment track of the psd. A comment on a frame can contain multiple comma separated marks, each with the syntax:
  <mark_type> [label]
  Examples of complete comment tracks (first number is frame number):

  0   "start intro"
  20  "stop intro, start outro"
  40  "remove"

  0   "start intro"
  20  "start loop_begin"
  80  "jump_to loop_begin, start outro"
  100 "remove"
* Fixed bug where usage of chroma key was never reset in the shader.