Show full screen windows with WS_POPUP style in wxMSW.

This doesn't make any difference for most windows but fixes a problem with
wxGLCanvas windows using stereo support of Nvidia cards and generally it does
make sense to use WS_POPUP with full screen windows as they are definitely not
overlapped, so just use this style unconditionally.

Closes #15434.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74818 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/msw/toplevel.cpp

@@ -1100,6 +1100,12 @@ bool wxTopLevelWindowMSW::ShowFullScreen(bool show, long style)
 
         newStyle &= ~offFlags;
 
+        // Full screen windows should logically be popups as they don't have
+        // decorations (and are definitely not children) and while not using
+        // this style doesn't seem to make any difference for most windows, it
+        // breaks wxGLCanvas in some cases, see #15434, so just always use it.
+        newStyle |= WS_POPUP;
+
         // change our window style to be compatible with full-screen mode
         ::SetWindowLong(GetHwnd(), GWL_STYLE, newStyle);