This migrates TextNode::text_for_rendering() to Utf16String and deals
with the fallout. As a consequence, this effectively reverts commit
3df83dade8.
The layout test expecation file updates are because we now express text
lengths and offsets in UTF-16 code units.
The selection-over-multiple-code-units expectation file update actually
represents a correctness fix. Our result now matches Firefox.
We generated `PaintableFragment`s with a start and length represented in
UTF-8 byte offsets, but failed to consider that the offsets in a
`DOM::Range` are actually expressed in UTF-16 code units.
This is a bit of a mess: almost all web specs use UTF-16 code units as
the unit for indexing into text nodes, but we almost exclusively use
UTF-8 in our code base. Arguably the best thing would for us to use
UTF-16 everywhere as well: it prevents these mismatches in our
implementations for the price of a bit more memory usage - and even that
could potentially be optimized for.
But for now, try to do the correct thing and lazily allocate UTF-16 data
in a `PaintableFragment` whenever we need to index into it or if we're
asked to determine the code unit offset of a pixel position.
