[sld-parser] Parse color formats to hex-codes

[jansule]

PARSER

SLD

FEATUREREQUEST

Description:

Following the SLD specification the color property of a Symbolizer must be specified as hex-code. However, other styles (e.g. openlayers, mapbox) allow a multitude of formats e.g. words ('green'), hsl() and rgba().

The geostyler-style only specifies the property color as a string.

It would be nice if the SLD parser would be able to parse before mentioned formats to hex-codes so that we do not have to limit allowed formats for the other parsers.