Adds Units to Gradient LayerTree model

Author: swhitty

SwiftDraw/LayerTree.Builder.swift

@@ -213,6 +213,14 @@ extension LayerTree.Builder {
       gradient.stops = makeGradientStops(for: element)
     }
 
+    if element.gradientUnits == .userSpaceOnUse {
+      gradient.units = .userSpaceOnUse
+    }
+
+    guard gradient.stops.count > 1 else {
+      return nil
+    }
+
     return gradient
   }
 

SwiftDraw/LayerTree.Gradient.swift

@@ -35,7 +35,8 @@ extension LayerTree {
     var start: Point
     var end: Point
     var stops: [Stop]
-    
+    var units: Units = .objectBoundingBox
+
     init(start: Point, end: Point) {
       self.start = start
       self.end = end
@@ -60,5 +61,10 @@ extension LayerTree {
         self.opacity = opacity
       }
     }
+
+    enum Units {
+      case userSpaceOnUse
+      case objectBoundingBox
+    }
   }
 }