Set caret height correctly for cells inside math rows

Set caret height correctly for cells inside math rows

The code in 90cfe4ec3 only handled the cells which metrics are
computed directly, and missed those who were linearized inside a MathRow.

To fix this, we use the fact that all the positions in a math row have
the same height and make MathRow::metrics return a boolean indicating
whether it contains a caret for a given bufferview.

Fixes bug #11153.