09a48e Fix: Allow negative frame id in scene.setCell (scripting). (#345)

1 file Authored by Banbury 8 years ago , Committed by Keisuke Ogaki 8 years ago ,
    Fix: Allow negative frame id in scene.setCell (scripting). (#345)
    
    This fix is necessary for creating animations in Javascript from raster images

    with only one frame. For this Opentoonz expects a frame id of -2.

    A frame id of -1 is an empty frame.

    The original implementation onyl allows positive numbers.