In order to make the message handling available to windowless richedit
controls, the message handling must be in a function that can be
called from the ITextServices_TxSendMessage method.  This method will
never have a handle to a window to pass to RichEditWndProc_common in
order to get the editor with GetWindowLongPtrW, but passing the editor
will work (even if hWnd is NULL).

When the text is wrapped, the positions for all the runs, paragraphs,
and cells are already calculated and stored.  The only thing left to do
for painting is to offset them by the formatting rectangle and the
scroll position.

During wrapping there were three different heights that were being
stored, with only one of them being done correctly.  The other ones
failed to incorporate the height of the paragraph or row, so ended up
being incorrect.