rework how clipping rects are calculated. fixes #149

[msparkles]

reopened from #210 (we moved the repo from forking from automancy's fork to forking from upstream instead because of weird Github shenanigans)

[msparkles]

#210 (comment)

Ran into a case where I'd love to have this feature! I haven't managed to untangle our use of yakui viewports in our game yet though and that's still a case where this PR doesn't intersect.

A related question for this: do you have any ideas for how we can bump widgets around so that they stay on screen? One use for this PR is for popping up boxes from within a heavily clipped context, like for tooltips. You also probably want to ensure those tooltips don't run off the edge of the screen!

@LPGhatguy we did something about this but we don't know how we feel about it. we consulted our partner who was visiting the other day for opinion and she said "i don't think the clipping logic should do that much work", but also making the Tooltip example in the current yakui model of layouting logic is just either asinine or impossible to be featureful enough.

Well, she also pointed out that the example's behaviour isn't what most tooltips do, but the rationale here is "it's an example, one that's meant to showcase capabilities" but if one were to make the same thing completely in widget code it would simply be impossible (or suboptimal) (or requires a lot of hacky patches to yakui, guess what we ended up with in our fork before lol)