compile_value(): bugfix, check for @type 'tag_value'
Initial support for Mapnik 'expr-v2' branch

- Mapnik expr-v2 can read colors for LineSymbolizers, PolygonSymbolizers and TextSymbolizers (with ExpressionFormat) from database columns. Therefore the values for 'color', 'casing-color', 'fill-color', 'text-color' and 'text-halo-color' may result from an eval() statement. Also resulting .mapnik-files will be smaller.
eval-functions: rgb() and rgba()
eval-functions, color: deluma(), lighten()