[cssom-view] Range.getClientRects() when the range contains part of a grapheme cluster

We recently figured out that the spec[1] isn't clear when the range
contains part of a grapheme cluster. Our behavior for the case was recently
changed due to some other improvements, which lead to a discussion of the
right behavior of getClientRects() and getBoundingClientRect().

Could we define cases such as:
1. What to do when the range contains part of a grapheme cluster.
2. What to do when the range contains part of a surrogate pair.
3. What to do when the range contains part of a ligature.

[1] https://drafts.csswg.org/cssom-view/#extensions-to-the-range-interface

/koji