wxBitmap with cleared alpha channel doesn't keep its alpha flag any more.

Explicitly reset wxBitmap alpha flag after clearing its alpha channel to
ensure that we don't treat it as having alpha after going to all the trouble
of ensuring that it doesn't/

See #14403.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@75588 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/msw/bitmap.h

@@ -168,7 +168,8 @@ public:
     // these functions are internal and shouldn't be used, they risk to
     // disappear in the future
     bool HasAlpha() const;
-    void UseAlpha();
+    void UseAlpha(bool use = true);
+    void ResetAlpha() { UseAlpha(false); }
 
     // support for scaled bitmaps
     virtual double GetScaleFactor() const { return 1.0; }