This change makes it so that tranforms (including CSS transform and
scrolling) are properly accounted for when calculating glyph indices.
Containing block sizes are moved to `BaseFragment`. This should reduce
the size of the StackingContextTree a little.
Testing: A Servo-specific WPT-style test is added for this change.
Signed-off-by: Martin Robinson <mrobinson@igalia.com>
Co-authored-by: Oriol Brufau <obrufau@igalia.com>