Really fix source coordinates handling in wxDC::Blit() in wxMSW.

Really fix source coordinates handling in wxDC::Blit() in wxMSW.

It turns out that the changes r71028 were unnecessary (and actually harmful)
in most cases, they're only needed when a DIB is used as a source DC. So move
the manual coordinates adjustments to the branch of code using StretchDIBits()
and don't do it anywhere else.

Also don't list this as an incompatible change as wxMSW actually already
worked as the other ports in the majority of cases and list it as a simple bug
fix instead.

Closes #14188.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@71095 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775