tree/container: ensure pixman rect is valid in container_arrange_title_bar()

[emersion]

Fixes "Invalid rectangle passed" errors printed by Pixman.

[mstoeckl]

The errors mentioned can be triggered by setting the titlebar horizontal padding to some large value, with e.g. swaymsg titlebar_padding 500 5.

This PR does fix the invalid rectangle warnings. However, passing a width of 0 to sway_text_node_set_max_width does not, as I'd expect, hide the title text node, and the node still renders (at full width) off to the right of the titlebar. For example:

Adding the following line fixes the edge case for me, but I don't know if it's the right way to handle the issue.

wlr_scene_node_set_enabled(node->node, alloc_width > 0);

[Nefsen402]

The reason why sway_text_node_set_max_width() with 0 doesn't work is because it will internally generate a wlr_fbox with 0 area. wlr_scene special cases these rects and interprets them as having a source box of the entire input texture.

Nevermind, it special cases 0 width as having no max with but I think the fbox concern is real.

As a fixup, this MR LGTM.

[Nefsen402]

Fixed up the max_width thing with #8140