This ensures that the plaintext serializer doesn't use the preformatted
text code path if we have encountered a pre-wrap element that
Thunderbird uses (which means setting white-space: pre-wrap and width:
NNch on the body element.)
It also ensures that we use 0 as the wrap column number passed down to
the plaintext serializer, instead of -1, which this code seems to be
unable to handle properly.
This patch handles most of the call sites for these allocations except
for a few where I added TODO comments with some information. Handling
those places may require reworking lots of code, so I prefer to not do
that here.
