Fix incorrect scroll positions used in SetScrollbars().

Fix incorrect scroll positions used in SetScrollbars().

SetScrollbars() passed its input positions expressed in scroll units to
CalcUnscrolledPosition() which takes positions in pixels. This was definitely
wrong so don't do this and perform the conversion from scroll units to pixels
in SetScrollbars() itself for clarity instead.

It's not clear what concrete bugs, if any, does this fix as the calculated
positions are almost never used anyhow but the old code was obviously
incorrect and the new version has a chance of not being wrong so it's already
an improvement.

Closes #9988.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@70443 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775