When an element creates a stacking context (e.g. via position: relative
with z-index), its text fragments were not being hit tested. This was
because PaintableBox::hit_test() returns early when it has a stacking
context, and StackingContext::hit_test() only iterated child paintables,
not the stacking context root's own fragments.
Fix this by extracting fragment hit testing into a new method
hit_test_fragments() on PaintableWithLines, and calling it from
StackingContext::hit_test() when the stacking context root is a
PaintableWithLines.
