wlr_output: Set current_mode during new custom mode

When setting a custom mode current_mode will not be reset. That means
that next time the compositor tries to reset back from the custom mode,
it will compare against the stale current_mode and unset
WLR_OUTPUT_STATE_MODE erroneously.

types/output/output.c

@@ -358,6 +358,7 @@ static void output_apply_state(struct wlr_output *output,
 			}
 			break;
 		case WLR_OUTPUT_STATE_MODE_CUSTOM:
+			output->current_mode = NULL;
 			width = state->custom_mode.width;
 			height = state->custom_mode.height;
 			refresh = state->custom_mode.refresh;