Avoid rounding errors in wxMSW wxDC scaling code.

Avoid rounding errors in wxMSW wxDC scaling code.

Multiply by scale factor when it's > 1 and divide by it when it's < 1 to avoid
rounding errors. By choosing whether to increase the viewport or window
extents we increase precisions without any apparent negative effects (at least
under Windows NT where the coordinates in 2^27 range are supported, but even
under Windows 9x it's not clear if the old code was better as while we never
overflowed the viewport extents, we could overflow the window ones easily for
small zoom factors).

Closes #9554.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@67623 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775