gdkwindow: Don't enforce a native window for a custom visual

gdkwindow: Don't enforce a native window for a custom visual

In the ancient X days you could have Xservers that had multiple active windows, like
one truecolor and one 8bit palette. Then most apps ran in 8bpp but a single window
would use truecolor. This is done by specifying different visuals for the windows.

To make this work we ensured that a window with a visual different from its parent
gets a native subwindow, so that X can tell the hardware to do its magic.

These days the only real time we get two different visual is when one is a rgba visual
and the other is not. So, the code to check this doesn't really do anything but
get in the way when someone accidentally manages to not get a rgba visual on
a child window (see bb7054b508bfe41eb3e7f27496d4c2eb3a2b2f7a). So, to avoid
such errors we just remove the "different visual than parent" check.