* Add to_computed method to calculate `calc()` value with parent size,
if parent container size is `None`, the result will be `None`.
* Add from_option method for `MaybeAuto`, to construct from
`Option<Au>`.
* Update some test case.
The NoCalcLength::Absolute variant has been rewritten to accommodate
other units than Au with the new AbsoluteLength enum. This avoids loss
of precision for some operations.
The conversion from floating point absolute lengths to integer
application unit values adopts the same clamping limits as Gecko.
Correct viewport test references
These references were using an incorrect value for the expected
container width. Now that WebRender is no longer clipping to stacking
context boundaries the rendered width is correct (100% of viewport
width = 240px).
