Allow creating DIBs storing pixels in non-premultiplied format in wxMSW.

Allow creating DIBs storing pixels in non-premultiplied format in wxMSW.

wxDIB assumed that MSW always uses bitmaps in pre-multiplied pixel format but
this turns out to be wrong: some Windows functions such as ImageList_Draw()
or CreateIconIndirect() apply pre-multiplication internally and so must be
given data in non-pre-multiplied format on input.

This commit adds a possibility to create wxDIB in such format but doesn't use
it anywhere yet, this will be done in subsequent commits.

See #9050.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65959 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775