Fix alpha after drawing a bitmap with alpha on a purely RGB bitmap in wxMSW.

Previously, the area of the bitmap outside of the rectangle covered by the
bitmap being drawn became completely transparent, "losing" the original bitmap
contents. This was due to some pixels of the bitmap having non-0 alpha value
after AlphaBlend() call, as it updates both the colours and alpha of the
destination. As there doesn't seem to be any way to prevent it from doing
this, just reset alpha back to 0 after calling it to avoid the problem.

Closes #14403.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@75566 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
Fix handling of bitmaps with alpha channel in wxMSW wxImageList.

Don't use mask and alpha together, this results in visual artefacts and masks
are unnecessary with RGBA bitmaps anyhow.

The only potentially problematic remaining case is mixing bitmaps with alpha
and mask inside the same image list (as we need to indicate whether we use the
mask or not when creating it), but this should probably be rare and in the
meanwhile we can at least RGBA bitmaps with image lists, which includes doing
this implicitly when they are used as button bitmaps.

Also refactor wxBitmap code to extract part of CopyFromIconOrCursor() to allow
reusing it in the newly added MSWUpdateAlpha().

See #11476.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@75567 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775