render/texture: Emit signals on destroy and update

This allows compositors to track the texture life cycle.

Allows e.g. compositors using the GLES2 renderer to know
when the texture gotten via `wlr_gles2_texture_get_attribs()`
changes.

Signed-off-by: Guido Günther <agx@sigxcpu.org>

include/wlr/render/wlr_texture.h

@@ -24,6 +24,11 @@ struct wlr_texture {
 	uint32_t width, height;
 
 	struct wlr_renderer *renderer;
+
+	struct {
+		struct wl_signal destroy;
+		struct wl_signal updated;
+	} events;
 };
 
 struct wlr_texture_read_pixels_options {