+ implement offset

```
offset = "<number> | <percentage>"
```

include/mapnik/image_filter.hpp

@@ -417,9 +417,15 @@ void apply_filter(Src & src, colorize_alpha const& op)
 
     double step = 1.0/(size-1);
     double offset = 0.0;
-    BOOST_FOREACH( mapnik::color const& c, op)
+    BOOST_FOREACH( mapnik::filter::color_stop const& stop, op)
     {
-        grad_lut.add_color(offset, agg::rgba(c.red()/256.0,
+        mapnik::color const& c = stop.color;
+        double stop_offset = stop.offset;
+        if (stop_offset == 0)
+        {
+            stop_offset = offset;
+        }
+        grad_lut.add_color(stop_offset, agg::rgba(c.red()/256.0,
                                                   c.green()/256.0,
                                                   c.blue()/256.0,
                                                   c.alpha()/256.0));

include/mapnik/image_filter_grammar.hpp

@@ -25,17 +25,40 @@
 
 // boost
 #include <boost/spirit/include/qi.hpp>
+#include <boost/fusion/include/adapt_struct.hpp>
 // mapnik
 #include <mapnik/css_color_grammar.hpp>
 #include <mapnik/image_filter.hpp>
 // stl
 #include <vector>
 
+BOOST_FUSION_ADAPT_STRUCT(
+    mapnik::filter::color_stop,
+    (mapnik::color, color )
+    (double, offset)
+)
 
 namespace mapnik {
 
 namespace qi = boost::spirit::qi;
 
+struct percent_offset_impl
+{
+    template <typename T>
+    struct result
+    {
+        typedef double type;
+    };
+
+    double operator() (double val) const
+    {
+        double result = std::abs(val/100.0);
+        if (result > 1.0) result = 1.0;
+        return result;
+    }
+};
+
+
 template <typename Iterator, typename ContType>
 struct image_filter_grammar :
         qi::grammar<Iterator, ContType(), qi::ascii::space_type>
@@ -46,10 +69,12 @@ struct image_filter_grammar :
     qi::rule<Iterator, qi::locals<int,int>, void(ContType&), qi::ascii::space_type> agg_blur_filter;
     qi::rule<Iterator, qi::locals<double,double,double,double,double,double,double,double>,
              void(ContType&), qi::ascii::space_type> hsla_filter;
-    qi::rule<Iterator, qi::locals<mapnik::filter::colorize_alpha>, void(ContType&), qi::ascii::space_type> colorize_alpha_filter;
+    qi::rule<Iterator, qi::locals<mapnik::filter::colorize_alpha, mapnik::filter::color_stop>, void(ContType&), qi::ascii::space_type> colorize_alpha_filter;
     qi::rule<Iterator, qi::ascii::space_type> no_args;
     qi::uint_parser< unsigned, 10, 1, 3 > radius_;
     css_color_grammar<Iterator> css_color_;
+    qi::rule<Iterator,void(mapnik::filter::color_stop &),qi::ascii::space_type> color_stop_offset;
+    phoenix::function<percent_offset_impl> percent_offset;
 };
 
 }

include/mapnik/image_filter_types.hpp

@@ -98,16 +98,18 @@ struct hsla
     double a1;
 };
 
-struct colorize_alpha : std::vector<mapnik::color>
+struct color_stop
+{
+    color_stop() {}
+    color_stop(mapnik::color const& c, double val = 0.0)
+        : color(c),offset(val) {}
+    mapnik::color color;
+    double offset;
+};
+
+struct colorize_alpha : std::vector<color_stop>
 {
     colorize_alpha() {}
-    colorize_alpha(mapnik::color const& c0, mapnik::color const& c1)
-    {
-        this->push_back(c0);
-        this->push_back(c1);
-        // TODO: support multiple color-stops
-        // https://developer.mozilla.org/en-US/docs/CSS/linear-gradient
-    }
 };
 
 typedef boost::variant<filter::blur,

src/image_filter_grammar.cpp

@@ -57,7 +57,7 @@ image_filter_grammar<Iterator,ContType>::image_filter_grammar()
     using boost::spirit::ascii::string;
     using phoenix::push_back;
     using phoenix::construct;
-
+    using phoenix::at_c;
 #if BOOST_VERSION >= 104700
     using qi::no_skip;
     start = -(filter % no_skip[*char_(", ")])
@@ -111,11 +111,17 @@ image_filter_grammar<Iterator,ContType>::image_filter_grammar()
 
     colorize_alpha_filter = lit("colorize-alpha")[_a = construct<mapnik::filter::colorize_alpha>()]
         >> lit('(')
-        >> css_color_[push_back(_a, _1)]
-        >> +(lit(',') >> css_color_[push_back(_a, _1)])
+        >> (css_color_[at_c<0>(_b) = _1, at_c<1>(_b) = 0]
+            >> -color_stop_offset(_b)) [push_back(_a,_b)]
+        >> +(lit(',') >> css_color_[at_c<0>(_b) =_1,at_c<1>(_b) = 0]
+             >> -color_stop_offset(_b))[push_back(_a,_b)]
         >> lit(')') [push_back(_r1,_a)]
         ;
 
+    color_stop_offset = (double_ >> lit('%'))[at_c<1>(_r1) = percent_offset(_1)]
+        |
+        double_[at_c<1>(_r1) = _1]
+        ;
     no_args = -(lit('(') >> lit(')'));
 }