[TTML2] tts:{width,height} rename

The use of width and height as writing mode relative properties is
confusing. Change their names to ipd and bpd, abbreviations for inline
progression dimension and block progression dimension, respectively, and
document convention that width and height (as well as horizontal and
vertical) are always absolute and not writing mode relative. The only
exception being that 'height' in lineHeight remains writing mode relative,
i.e., specifies the nominal bpd of a line area.

Change image to use tts:extent instead of the former tts:{width,height} in
order to use absolute axes in expressing explicit image dimensions.

Addressed above comments in [1].

[1] https://dvcs.w3.org/hg/ttml/rev/69877acd9380