wpe: Don't crash when running on X11.
Don't assume the available EGL display is a wayland display - instead, check the the GStreamer GL context is EGL, and then use gst_gl_display_egl_from_gl_display to create a GstGLDisplayEGL from that, which also adds refcounting around the underlying EGLDisplay.
Fixes #1385 (closed)
I've been testing this patch locally for a while and seems OK, but I don't have other platforms to test on.