Fix #402 Use CCS Colour L4 definition for computed value of *-opacity

[dstorey]

No description provided.

[dstorey]

Notes:

• none of the CSS properties in pservers include a row for computed value, so I didn't change there. I just used the data type rather than number | percentage
• solid-opacity is not included in propidx
• flood-opacity is defined in the Filters spec. @dirkschulze: should that be updated to use alpha-value? it uses https://drafts.csswg.org/css-values-4/#typedef-number-percentage which is also number | percentage but is more general purpose (doesn't mention useful range being between 0 and 1)
• doesn't this need a change log entry or tests? Feels more like tightening up to expected platform conventions.

[dirkschulze]

@dstorey It would be great if you could open an issue on https://github.com/w3c/fxtf-drafts/issues so that I won't forget about it.