Make raw pixel access work even with DDBs under MSW.

This is inefficient, because we need to copy pixels to and from them, but
better than silently doing nothing.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@75438 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
Mention that raw pixel access is efficient only with DIBs under MSW.

While accessing DDBs (now) works too, it's inefficient and should be avoided.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@75439 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775