generate
Generates a list of acceptable preview sizes. Preview sizes are not acceptable if there is not a corresponding picture size of the same aspect ratio. If there is a corresponding picture size of the same aspect ratio, the picture size is paired up with the preview size.
This is necessary because even if we don't use still pictures, the still picture size must be set to a size that is the same aspect ratio as the preview size we choose. Otherwise, the preview images may be distorted on some devices.