We don't currently support values 1 (centimeters), 2 (inches) or 3 (DAR),
only the default value 0 (pixels) which doesn't need to be written.

The fate refs are updated as unknown SAR is now signaled in the output
files with the addition of the new element.
Reviewed-by: Carl Eugen Hoyos <ceffmpeg@gmail.com>
Signed-off-by: James Almer <jamrial@gmail.com>