Fix wxHtmlWindow to correctly decide whether to show scrollbars.

Fix wxHtmlWindow to correctly decide whether to show scrollbars.

wxHtmlWindow::CreateLayout()'s algorithm was both naive (not properly
accounting for scrollbar visibility changes during layout) and broken
(incorrectly rounding when computing scroll steps and adding
GetCharHeight() value to the height in an attempt to mitigate the
harm).

This algorithm should properly for scrollbars in all situations.
Rounding is done in such way that the content is fully viewable, while
at the same time not showing scrollbars needlessly.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@63483 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775