encourage use of BoxConstraints::constrain in all layout methods

[cmyr]

a general note; I think it's good to call bc.constrain(size) anytime you return from layout, as a way of ensuring all returned sizes are appropriate for the constraints.

Originally posted by @cmyr in #845

[cmyr]

Another thing that has occurred to me is that we could track when this is and isn't called, and log when it isn't. I'm not sure this makes sense, but it is an option.

[xStrom]

Track it in WidgetPod? Might as well just call it in WidgetPod's layout then.

[luleyleo]

I wonder if there is any valid use case for a widget to violate its constraints?
Considering how mouse / touch input operates in druid it seems like violating constraints would actually break events in a nasty way. Probably enforcing constraints in WidgetPod::layout and logging a warning when the constraints are not met would make sens.

[cmyr]

yea, I think violating constraints is an error.