Fix wxStaticBitmap best size calculation in wxMSW.

Take the control borders into account by overriding DoGetBestClientSize()
instead of DoGetBestSize().

Also invalidate the default best size computed before we have any valid image
when setting the initial image in Create().

Closes #4099.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@71380 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/msw/statbmp.h

@@ -58,7 +58,7 @@ public:
     virtual bool CanApplyThemeBorder() const { return false; }
 
 protected:
-    virtual wxSize DoGetBestSize() const;
+    virtual wxSize DoGetBestClientSize() const;
 
     // ctor/dtor helpers
     void Init() { m_isIcon = true; m_image = NULL; m_currentHandle = 0; }