Allow configuring LSM to setup wayland screen name via WAYLAND_DISPLAY_LSM

[ericblade]

• This allows using Wayland as luna-surfacemanager's output, so lsm becomes a forwarding or nested compositor.

  You can set

XDG_RUNTIME_DIR=/path/to/output-compositor/xdg/run 
WAYLAND_DISPLAY="wayland-0" # name of display on output compositor 
WAYLAND_DISPLAY_LSM="wayland-1" # name of display for LSM apps 
WEBOS_COMPOSITOR_PLATFORM=wayland 

... then run surface-manager, and surface-manager's output will appear in the host wayland compositor. webOS apps will need to have WAYLAND_DISPLAY set to wayland-1 or they will end up directly on the host compositor, instead of inside LSM.

• Move checkDaemonFiles into constructor, replacing the setSocketName() call, as setSocketName() would default to using WAYLAND_DISPLAY, which would then crash because the host compositor is already using that.
• Check for null on QGuiApplication::primaryScreen() before calling name(), so that the application will crash with a proper fatal error from Qt when no screens are found

[CLAassistant]

All committers have signed the CLA.

[ericblade]

Switching this to draft, as although I had it working in a test build, once I built a complete image, it no longer worked, so I'm not sure if I messed up something in this patch or if i'm missing some other piece of what I built (I think it's hardware driver related, but i'm leaving this patch as draft until i prove that out)

[ericblade]

Confirming that this functions. My issue previously was with some bad mesa configuration selection.

Thank you for your consideration.