xwayland/glamor/gbm: Use EGL_NO_CONTEXT with EGL_NATIVE_PIXMAP_KHR

This is required per
https://www.khronos.org/registry/EGL/extensions/KHR/EGL_KHR_image_pixmap.txt .

Mesa hasn't enforced it, but it will soon:
https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/454

Fixes: 2f113d68f6 ("xwayland: Add glamor and DRI3 support")

hw/xwayland/xwayland-glamor-gbm.c

@@ -237,7 +237,7 @@ xwl_glamor_gbm_create_pixmap_for_bo(ScreenPtr screen, struct gbm_bo *bo,
 #endif
     {
         xwl_pixmap->image = eglCreateImageKHR(xwl_screen->egl_display,
-                                              xwl_screen->egl_context,
+                                              EGL_NO_CONTEXT,
                                               EGL_NATIVE_PIXMAP_KHR,
                                               xwl_pixmap->bo, NULL);
     }