implement layer level AUTO_LENGTH override to support AUTO play for all producers

[TomKaltz]

This PR adds layer level AUTO_LENGTH to PLAY, LOADBG and LOAD to override auto play behavior at the layer level. This allows auto play functionality with infinite length producers (decklink, bluefish, looping content, etc.). Previously, infinite length producers couldn't use timed auto play since their nb_frames() returns infinity. With this change, users can now specify AUTO_LENGTH with any producer to override nb_franes. The implementation tracks separate auto_length overrides for foreground and background producers at the layer level.

This PR also adds a layer level CALL FRAMES_LEFT that allows you to query, set and clear the AUTO_LENGTH overide to augment auto play functionality at runtime. CALL 1-0 FRAMES_LEFT 200 will change the AUTO_LENGTH override so that the auto transition will occur 200 frames from when the command was executed, effectively starting a countdown clock of the specified frames. It will also set AUTO play to true if there is a background producer loaded. CALL 1-0 FRAMES LEFT CLEAR will remove any AUTO_LENGTH override and also disable auto play.

notable autoplay behavior

• If a producer's actual length is shorter than the supplied AUTO_LENGTH (ffmpeg producer with short file or IN, OUT creating a shorter than AUTO_LENGTH clip), then the last frame is repeated until LENGTH frame count is achieved.
• You can use IN, OUT(or LENGTH), LOOP and AUTO_LENGTH together with ffmpeg producer to create a custom looping clip that will loop until AUTO_LENGTH frame count is achieved.

closes #1644