Elaborate scaling as pertains to tts:fontSize.

[nigelmegitt]

One of two points arising from w3c/ttml1#301:

The mechanism for scaling to achieve a non-square em square needs more elaboration, for example a note:

Note: Presentation processors are expected to scale glyphs so that the height of the em square as defined in the font resource matches the computed font size height, using an appropriate algorithm for the font. If the computed font size width differs from the computed font size height presentation processors additionally need to scale the em square anamorphically by the ratio of the width to the height.