Add rounding to the size given to BoxConstraints::constrain.

[xStrom]

This PR is a continuation of the pixel grid work started in #669. Adding Size::expand to BoxConstraints::constrain seems to go a long way in solving this. It solves the issue for Label but also in more subtle places like Switch which depend on theme variables by doing things like this:

let width = env.get(theme::BORDERED_WIDGET_HEIGHT) * SWITCH_WIDTH_RATIO; // 24 * 2.75

Doing it in BoxConstraints::constrain will also help with custom widgets that users might unknowingly make fractional.

I reviewed all current widgets in druid and this change doesn't seem to have any other effects at this point.

[cmyr]

Excellent, thanks.