Fix conversion of 32 bit ARGB bitmaps to wxImage in wxMSW.

Fix conversion of 32 bit ARGB bitmaps to wxImage in wxMSW.

wxDIB::m_hasAlpha can't be trusted when the DIB was loaded from a file so
don't rely on it in wxDIB::ConvertToImage(). Instead, suppose that 32 bpp
bitmaps do have alpha channel and only get rid of it at the end of conversion
if it turns out that all alpha values were 0.

Closes #10133.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@65898 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775