g-s doesn't start Xwayland >= 1.20 with -eglstream when using eglstreams

This had slipped my mind until now, but luckily it's a rather small thing to fix. When using mutter with EGLStreams, we should also enable EGLStream acceleration in Xwayland by passing -eglstreams.

There's one issue with this that ofourdan brought up, mainly that Xwayland versions before 1.20 won't support the -eglstream option. I would think this should be fairly easy to solve: if Xwayland -eglstream fails to start up (since the unkown arg will make it fail immediately), just start it again without the -eglstream arg.

Additionally as a note: there seemed to be a bit of confusion with some gnome developers on why Xwayland doesn't detect this automatically. I had originally intended for it to, but I realized that:

• We can reliably detect the presence of EGLStream devices, but not GBM (as mesa doesn't support the EGLDevice extensions)
• Passing -eglstream explicitly seemed like a better solution, as it's more important that we use eglstreams whenever mutter is using it (and mutter always knows what rendering backend it's using!) as opposed to whenever there's an eglstreams device present on the system. Currently mutter defaults to eglstreams above gbm always, but I don't want to make assumptions about other wayland compositors that may not follow the same behaviour.