Fix line-height being capped due to naive handling of negative ascent.
Because ascent and descent can be negative or positive depending on the writing mode, the intial code attempted to regard them somewhat agnostic until it was time to calculate the precise offset. This however capped the offset to 0, and made it impossible to have negative line-offsets.
The code now considers negative values appropriately.
(cherry picked from commit b90110d3)