Add bitmaps in non-premultiplied format to wxImageList in wxMSW.

Add bitmaps in non-premultiplied format to wxImageList in wxMSW.

ImageList_Draw() applies pre-multiplication to the bitmap itself (as can be
seen by experimenting with this or reading WINE sources for it) and so the
image list must store bitmaps with data in non-pre-multiplied format to avoid
doing it twice.

Do it by converting wxBitmaps passed to wxImageList::Add() and Replace() to
wxImage and then to non-pre-multiplied DIBs. This is obviously very
inefficient but at least results in correct appearance of images drawn by
wxImageList so it's a step forward.

Closes #9050.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65960 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775