Simplify and correct bugs in wxMSW wxScrollBar message handling.

Simplify and correct bugs in wxMSW wxScrollBar message handling.

For some reason we computed scroll increment from the native message and not
the new position directly which is actually simpler and probably would have
never resulted in a bug which exchanged the meanings of SB_TOP and SB_BOTTOM.

Get rid of nScrollInc and just update the position variable directly.

Closes #11741.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@63587 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775