This function was difficult to understand as it did two quite different
things depending on the value of its "dont_render" argument and using it
also made CountPages() logic more complicated than necessary.
Simplify the code by splitting Render() into FindNextPageBreak(), which
is used by CountPages(), and Render() itself, which doesn't need to deal
with pagination at all as it's either already provided the page start
and end positions or can just assume that everything fits on a single
page (the latter is the case for the headers and footers renderer).
This is a notionally backwards-incompatible change, but no code using
wxHtmlDCRenderer could be found in the wild and the new API is so much
simpler that it seems to be worth switching to it.
e01892c669