Adwaita: avoid border clash on transitioning adiacent flat buttons

Flat buttons gets the button decoration on hover, while transitioning
the decorations of adiacent flat buttons are both shown (one fading in
and the other fading out) so the borders clashes, since normally there's
no spacing between them, to avoid it the transition on the normal state
is set to none and added back to the hover state, so the decoration
won't fade out. To make the transition more evident the duration is
increased.
X11: Fix crash in gdk_x11_gl_context_texture_from_surface

We need to look at the impl_window for the gl rendering, not
the subwindow we're rendering into.
Adwaita: tweak list row transition

Thanks to Carlos Soriano expertise.
Adwaita: list row hover color

Now it's based of fg color, so the list row gets darker on the
bright variant and brighter on the dark variant, similarly to what
we do for spinbutton buttons.