TTML writer should not output cell units

[palemieux]

In IMSC, "c" units are only permitted on the ebutts:linePadding property.

Instead TTML writer should allow the user to specify that rh and rw units are used.

[palemieux]

rh and rw units cannot replace c units directly since the value of c depends on whether c is evaluated in a horizontal or vertical context. In the case of padding, for example, this cannot be determine without determining the computed value of tts:writingMode, which itself requires determining the region with which the element is associated.

It might therefore make more sense to limit the model such that only c units can be used with ebutts:linePadding, i.e. enforcing the IMSC model.