backend: Fix minor comment
window-x11: Update the input region after setting the client rect

We clip the input region to the client rect, so the client rect should
be up to date before we fetch the input region.

This fixes popup windows not working in GTK+2 under Wayland.

We should also update the shape / input regions when the window is
reconfigured for a complete fix, so that making an O-R window bigger
doesn't confuse mutter, but let's leave that to a future commit.