Add support for user defined displayAspectRatio

[xchange11]

If an EBU STL file was created for video content with an aspect ratio of 4:3 and this video content is rendered in 16:9 without cropping, a subtitle positioned on the far left or far right of the mapped Teletext grid in the IMSC output will be partially rendered on the black bars (the same can happen with scc files). To avoid this, it would be good if the user could set the aspectRatio as a parameter. The semantics should be as defined for #aspectRatio in IMSC 1.0.1 and #displayAspectRatio in IMSC 1.1 and higher (#aspectRatio is permitted-deprecated since IMSC 1.1). The result would be that the root container region of the IMSC document is mapped to a rectangular area within the image frame such that:

1. the ratio of the width to the height of the rectangular area is equal to the defined ratio,
2. the center of the rectangular area is collocated with the center of the image frame,
3. the rectangular area is entirely within the image frame, and
4. the rectangular area has a height or width equal to that of the image frame.