render/egl: add support for EGL_EXT_device_drm_render_node

This EGL extension has been added in [1]. The upsides are:

- We directly get a render node, instead of having to convert the
  primary node name to a render node name.
- If EGL_DRM_RENDER_NODE_FILE_EXT returns NULL, that means there is
  no render node being used by the driver.

[1]: https://github.com/KhronosGroup/EGL-Registry/pull/127

include/wlr/render/egl.h

@@ -43,6 +43,7 @@ struct wlr_egl {
 
 		// Device extensions
 		bool EXT_device_drm;
+		bool EXT_device_drm_render_node;
 	} exts;
 
 	struct {