output: reset hardware_cursor on failure

The DRM backend's set_cursor function always return true if the
buffer is NULL. If using a NULL cursor's buffer on startup, the
wlr_output_cursor will be marked as a hardware cursor. If the
cursor later gains a non-NULL buffer and the DRM backend rejects
that buffer, the cursor will remain marked as a hardware cursor,
despite the backend not displaying it as such. As a result, the
cursor will not be displayed at all. Fix this by always resetting
the hardware_cursor field in output_cursor_attempt_hardware().

types/output/cursor.c

@@ -280,6 +280,8 @@ static bool output_cursor_attempt_hardware(struct wlr_output_cursor *cursor) {
 		return false;
 	}
 
+	output->hardware_cursor = NULL;
+
 	struct wlr_texture *texture = cursor->texture;
 
 	// If the cursor was hidden or was a software cursor, the hardware