Factor out duplicated code for bitmap scaling in wxArtProvider.

Move duplicated code responsible for rescaling bitmaps in wxArtProvider to the
shared wxArtPrvider::RescaleBitmap() method.

Additionally, make this new method work even in wxUSE_IMAGE==0 case.

See #17071.

See https://github.com/wxWidgets/wxWidgets/pull/64.

include/wx/artprov.h

@@ -188,6 +188,9 @@ public:
     // the topmost provider if platform_dependent = false
     static wxSize GetSizeHint(const wxArtClient& client, bool platform_dependent = false);
 
+    // Rescale bitmap (used internally if requested size is other than the available).
+    static void RescaleBitmap(wxBitmap& bmp, const wxSize& sizeNeeded);
+
 protected:
     friend class wxArtProviderModule;
 #if wxUSE_ARTPROVIDER_STD