Fix wxMenuItem::SetBitmap infinite recursion differently.

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