Calculate line position for cues with their snapToLines flag set.

- This supports horizontal text only. It needs a few tweaks
to fully support vertical text.
- The test suite has been altered to balance the computed values of
processing model tests since they will be different on different
platforms. This means calculating offsets based on differences in
computed line heights and applying the offsets to the different
CSS position values.
Finish algorithm to position boxes when their snapToLines flag is not set.