Add smooth-algorithm to line symbolizer (PR #4031)

Author: dch0ph

4.2.0/reference.json

@@ -677,6 +677,16 @@
                 "range": "0-1",
                 "doc": "Smooths out geometry angles. 0 is no smoothing, 1 is fully smoothed. Values greater than 1 will produce wild, looping geometries."
             },
+            "smooth-algorithm": {
+                "css": "line-smooth-algorithm",
+                "type": ["basic",
+                         "adaptive"
+                        ],
+                "expression":true,
+                "default-value": "basic",
+                "default-meaning": "The geometry will be simplified using the original non-adaptive algorithm.",
+                "doc": "Set the alogorithm used to smooth geometries. `adaptive` will increase the number of interpolation points at sharper bends, leading to more realistic smoothing."
+            },
             "offset": {
                 "css": "line-offset",
                 "type": "float",