Instead of checking for the exact variant type match, call the new
IsCompatibleVariantType() virtual function, which still does the same
check by default, but can be overridden to allow other types as well if
they're accepted by the renderer.
This will be soon used to allow accepting both wxBitmap (which must
still be accepted for compatibility) and wxBitmapBundle (which is the
simplest way to support high DPI) in wxDataViewBitmapRenderer at once.
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