Don't consider fully opaque bitmaps as having alpha in wxMSW.

Remove the alpha channel from the DIB we create not only if all of its pixels
are fully transparent but also if they are all fully opaque. This prevents us
from erroneously creating bitmaps with alpha channel when none is needed nor
expected.

Closes #13056.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@67287 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775