FFMpegReader: Add an option to ignore the input SAR

Sometimes, we may want to drop thhis information.

Signed-off-by: Derek Buithenhuis <derek.buitenhuis@gmail.com>
FFMpegReader: Add an option for the user to set a custom SAR

This will only be enabled it "Keep Input SAR" is unchecked.

Signed-off-by: Derek Buithenhuis <derek.buitenhuis@gmail.com>