These functions are unsound.  They would only work if called before
the registration of RsvgHandleClass (e.g. before the first
instantiation of an RsvgHandle), as the global value they set is what
is used for g_object_class_install_property(..., "dpi-x") and "dpi-y".

We maintain their behavior, but instead recommend that callers use
rsvg_handle_set_dpi() or rsvg_handle_set_dpi_x_y().