Implement shm buffer surface attach interface

Implement surface_attach method. This is called when a client attaches an shm
buffer with wl_surface_attach().

Implement the GLES2 interface for attaching shm buffers. This creates an opengl
texture with the shm buffer contents for the surface.

This commit also includes some working code to render the surfaces onto the
screen for demonstration purposes.

examples/compositor.h

@@ -7,6 +7,7 @@ struct wl_compositor_state {
 	struct wl_global *wl_global;
 	struct wl_list wl_resources;
 	struct wlr_renderer *renderer;
+	struct wl_list surfaces;
 };
 
 void wl_compositor_init(struct wl_display *display,