bug: RuntimeError: memory access out of bounds

[nwittstruck]

Description

When a user creates more than ~ 160 images (26 seconds at 6 fps), Safari crashes with an RuntimeError: memory access out of bounds.
The reson for that is that we run ffmpeg separately for each image conversion. We should batch this and have ffmpeg run once for all images.

Checklist

• Larger clips can be created
• Run ffmpeg once for all image conversions
• Make sure import still works, as this uses webm files