Don't call SetBitmap from GetHBitmapForMenu but instead handle possibly
needed bitmap modifications earlier on during SetBitmap. Allows for
GetHBitmapForMenu to be const and gets rid of the clumsy re-entry check
introduced in r76754.
Also check the bitmap for alpha presence instead of needlessly converting
the bitmap to an image and checking the latter for alpha.
git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@76781 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775