Remove use of `generateCGImagesAsynchronously`

[sindresorhus]

The API has been a constant PITA and it's super buggy.

Instead, we should use AVAssetReader and readerOutput.copyNextSampleBuffer().

I think it would be best to wrap up all the logic into a similar interface as generateCGImagesAsynchronously, but instead of accepting time points, it would accept FPS.

Some inspiration here: https://github.com/twostraws/ControlRoom/blob/659c5d5f3ab982ea54c9346aec9b700c147112b5/ControlRoom/Extensions/AVAssetToGIF.swift

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This issue requires advanced Swift knowledge.

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You can use this to get the CGImage from the pixel buffer:

import VideoToolbox

extension CVPixelBuffer {
    var cgImage: CGImage? {
        var cgImage: CGImage?
        VTCreateCGImageFromCVPixelBuffer(self, options: nil, imageOut: &cgImage)
        return cgImage
    }
}