Rework signal connection in preparations for deinterlacing.

Rework signal connection in preparations for deinterlacing.

Several improvements:

 - Hold three fields in history instead of one.
 - Make input_frames exact; no more chains holding on to input frames
   that they do not actually use for anything (so they can be released
   sooner).
 - Much less roundabout code to give the textures to the input;
   the LiveInputWrapper will now simply fetch them from the mixer itself.