libweston: Drop requirement of setting mm_width/mm_height in backends

They were required for transitional phase in order not to
break previous weston_output_init(). Now, they can even
be initialized on enable, or left with defaults if backend
doesn't support them.

Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Signed-off-by: Armin Krezović <krezovic.armin@gmail.com>

libweston/compositor.c

@@ -4514,9 +4514,6 @@ weston_output_enable(struct weston_output *output)
 	if (!wl_list_empty(&c->output_list))
 		x = iterator->x + iterator->width;
 
-	/* Make sure the width and height are configured */
-	assert(output->mm_width && output->mm_height);
-
 	/* Make sure the scale is set up */
 	assert(output->scale);