Allowing users to choose the output backend of weston

[taoky]

Currently, x11docker will start weston with x11 backend when DISPLAY is set ($Hostdisplay is not null): https://github.com/mviereck/x11docker/blob/93a2793e62a9fd5e5bb0ed9ffd67467463ed8003/x11docker#L4030-L4034

However, this will also start weston in x11 when in a wayland session with xwayland support. And unfortunately, I got this bug (I submitted just now) of weston x11 backend, which makes the experience uncomfortable.

Although weston.ini(5) supports settings backends:

CORE SECTION
       The core section is used to select the startup compositor modules and general options.

       shell=desktop-shell.so
              specifies  a  shell  to load (string). This can be used to load your own implemented shell or one with
              Weston as default. Available shells in the /usr/lib/weston directory are:

                 desktop-shell.so

       xwayland=true
              ask Weston to load the XWayland module (boolean).

       modules=cms-colord.so,screen-share.so
              specifies the modules to load (string). Available modules in the /usr/lib/weston directory are:

                 cms-colord.so
                 screen-share.so

       backend=headless-backend.so
              overrides defaults backend. Available backend modules in the /usr/lib/libweston-11 directory are:

                 drm-backend.so
                 headless-backend.so
                 rdp-backend.so
                 wayland-backend.so
                 x11-backend.so

It seems that the backend set in command line takes higher priority than weston.ini:

https://github.com/mviereck/x11docker/blob/93a2793e62a9fd5e5bb0ed9ffd67467463ed8003/x11docker#L4137-L4141

Currently my workaround is to modify [ -n "$Hostdisplay" ] && Westonoutput="X" to [ -n "$Hostdisplay" ] && Westonoutput="WL", and set size manually in weston.ini (otherwise the window is too large when default option is wayland backend). I think that it can be fixed by:

1. If users don't provide their own weston.ini, keep current behavior.
2. Otherwise, check if backend is set inside weston.ini. If so, don't add --backend to $Compositorcommand.

[taoky]

And, unsetting DISPLAY when running x11docker also workarounds this, but breaks the clipboard sharing functionality.