mxf: Correctly support files from Pinnacle Thunder

mxf: Correctly support files from Pinnacle Thunder

Such files have IndexTableSegments which when parsed cover EditUnit
ranges like this:

 [0,1)
 [249,250)
 [249,377)
 [0,249)

where each interval is

 [IndexStartPosition, IndexStartPosition + IndexDuration)

This would be reduced to a sparse index like:

 [0,1), [249,250)

instead of the full range:

 [0,249), [249,377)

Signed-off-by: Luca Barbato <lu_zero@gentoo.org>