Add wxBitmap::GetDIPSize() and use it in wxBitmapBundleImplArt

This fixes the problem with wrong standard bitmaps size when using high
DPI for the main monitor with wxMSW, as GetScaledSize() used here since
31f2892200 (Avoid bitmap scaling in wxArtProvider::GetBitmapBundle(),
2021-12-17) was not the right function to use there.

Closes #19331.

src/msw/bitmap.cpp

@@ -1385,6 +1385,11 @@ double wxBitmap::GetScaleFactor() const
     return GetBitmapData()->m_scaleFactor;
 }
 
+wxSize wxBitmap::GetDIPSize() const
+{
+    return GetSize() / GetScaleFactor();
+}
+
 double wxBitmap::GetScaledWidth() const
 {
     return GetWidth();