Aligns the CSS <rgb()> and <rgba()> function behaviours

A DESCRIPTION OF THE PROBLEM :
Aligns the <rgb()> and <rgba()> function behaviours in CSS.java, as described in :
CSS Color Module Level 4
W3C Candidate Recommendation Snapshot, 5 July 2022
[6.1. The RGB functions: rgb() and rgba()](https://www.w3.org/TR/css-color-4/#funcdef-rgb)

- In this recommendation there is no difference between the two function behaviours but this is not the case in the JDK present implementation.
- Some other parts of the described syntax are not supported.

Details and the code to test the present implementation at https://github.com/scientificware/jdk/issues/16.