Implement Segment#isLinear() and Path#isPolygon().

src/path/Path.js

@@ -231,7 +231,15 @@ var Path = this.Path = PathItem.extend(/** @lends Path# */{
 	},
 
 	isEmpty: function() {
-		return this._segments.length == 0;
+		return this._segments.length === 0;
+	},
+
+	isPolygon: function() {
+		for (var i = 0, l = this._segments.length; i < l; i++) {
+			if (!this._segments[i].isLinear())
+				return false;
+		}
+		return true;
 	},
 
 	_apply: function(matrix) {

src/path/Segment.js

@@ -162,6 +162,16 @@ var Segment = this.Segment = Base.extend(/** @lends Segment# */{
 		// this.corner = !this._handleIn.isColinear(this._handleOut);
 	},
 
+	/**
+	 * Checks whether the segment has no handles defined, meaning it connects
+	 * two straight lines.
+	 *
+	 * @return {Boolean} {@true the segment is linear}
+	 */
+	isLinear: function() {
+		return this._handleIn.isZero() && this._handleOut.isZero();
+	},
+
 	_isSelected: function(point) {
 		var state = this._selectionState;
 		return point == this._point ? !!(state & SelectionState.POINT)