output-layer: add cropping support

Add a src_box state field. Use the SRC_* KMS props in the DRM
backend, reject the layers in the Wayland backend (for now, we can
support it later via viewporter).

types/output/output.c

@@ -832,6 +832,7 @@ bool wlr_output_commit_state(struct wlr_output *output,
 			wl_list_insert(output->layers.prev, &layer->link);
 
 			// Commit layer state
+			layer->src_box = layer_state->src_box;
 			layer->dst_box = layer_state->dst_box;
 		}
 	}