Add support for lesscss color operation functions

Description

Adds a bunch of lesscss.org color operation functions to help with common cartographic needs. Functions working with alpha have not been added, as alpha is managed as a separate entity in SLD and derivatives.

Added functions from the SPI registration file:

Environment

None

Assignee

Andrea Aime

Reporter

Andrea Aime

Triage

None

Fix versions

Priority

Medium
Configure