feat(AutoSizer): measure via getBoundingClientRect + add roundingMode prop for sub-pixel rounding control

[jonw09]

• The existing test suites (npm test) all pass
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As it stands right now the AutoSizer has an issue; any fractional heights/widths are rounded up. This means the child content is told it has more space to play with than the reality which can lead to overflow issues and the likes.

This change proposes a shift to using getBoundingClientRect which consumes the fractional height/width correctly and then rounds it according to the newly exposed roundingMode prop. By default, for backwards compatibility it will use ceil which rounds upwards.