render-device/gbm: Add getter for gbm_device

Will be needed, at least during a transition period, to e.g. deal with
gbm_surface's et. al. without having MetaRenderDevice learning about
render surfaces.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1854>

src/backends/native/meta-render-device-gbm.c

@@ -153,3 +153,9 @@ meta_render_device_gbm_new (MetaBackend     *backend,
                          "device-file", device_file,
                          NULL);
 }
+
+struct gbm_device *
+meta_render_device_gbm_get_gbm_device (MetaRenderDeviceGbm *render_device_gbm)
+{
+  return render_device_gbm->gbm_device;
+}

src/backends/native/meta-render-device-gbm.h

@@ -32,4 +32,6 @@ MetaRenderDeviceGbm * meta_render_device_gbm_new (MetaBackend     *backend,
                                                   MetaDeviceFile  *device_file,
                                                   GError         **error);
 
+struct gbm_device * meta_render_device_gbm_get_gbm_device (MetaRenderDeviceGbm *render_device_gbm);
+
 #endif /* META_RENDER_DEVICE_GBM_H */