[x11] Disable synchronized resizing for Xwayland < 1.21

In case Xwayland does not use multiple buffers, the currently attached
buffer is going to be destroyed if the frame window is resized. It may
render the previous and the current window pixmap invalid and thus result
in visual artifacts when an X11 client is being interactively resized.

In order to avoid the visual artifacts, this change disables support for
synchronized resizing for X11 clients if the version of Xwayland is less
than the version in which Xwayland started using multiple buffers, i.e.
1.21.

Differential Revision: https://phabricator.kde.org/D29250