Fix creation of wxBitmap from wxIcon in wxMSW: premultiply the pixels.

Fix creation of wxBitmap from wxIcon in wxMSW: premultiply the pixels.

wxBitmap in wxMSW is supposed to store its data in alpha-premultiplied format
but didn't do it when it was created from an icon (or a cursor), resulting in
wrong display of wxIcons with alpha channel when they were used for e.g. menu
items.

Fix this by ensuring that the data is always premultiplied. This is not the
best solution as in some cases (e.g. if this wxBitmap is added to wxImageList
later) we could need to undo this premultiplication later which is quite
inefficient but at least it's simple and straightforward.

Closes #11414.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@71385 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775