IMSC 1.1 at-risk features

Good morning/evening,

Per our meeting earlier today, below is the proposed disposition for the
features marked at-risk in IMSC 1.1:

- #shear. Specify as permitted.
- #lineShear. Specify as prohibited. The feature can be emulated using
#shear and mapping to CSS is not possible today. See [1].
- #length-root-container-relative. Specify as permitted, subject to the
constraints described at [2].

[1] https://github.com/w3c/imsc/issues/455
[2] https://github.com/w3c/imsc/pull/452

Best,

-- Pierre