Adding original frames causes glitches in loops

[henrikvilhelmberglund]

It seems like the code adding the original images causes visible glitches that are more obvious in loops.

Here is the current with a 30fps loop of 60 frames (multiplier 30):

In the start and end of this pingpong style loop the circle seems to grow.

Here's an alternative version with a 30fps loop of 60 frames (multiplier 32 with the original frames removed):

This looks smoother because there's not a huge jump between the first and second frame (same at the end). In the original version the circle seems bigger in the start/end frames and "jumping" to the right at the end and to the left at the start which seems weird.

Commenting out

        #output_frames[out_len] = frame0 # Start with first frame
        #out_len += 1

and

    # Append final frame
    #output_frames[out_len] = frames[-1:]
    #out_len += 1

and increasing the multiplier by 2 makes the loop smoother.

I have no real idea why this happens but something seems off about the switch from original images to interpolation.