[orx-shadestyles] n-point gradient shadeStyle and demos (#138)

orx-shade-styles/src/main/kotlin/LinearGradient.kt

@@ -11,7 +11,7 @@ import org.openrndr.math.Vector2
 class LinearGradient(
         color0: ColorRGBa,
         color1: ColorRGBa,
-        offset: Vector2,
+        offset: Vector2 = Vector2.ZERO,
         rotation: Double = 0.0,
         exponent: Double = 1.0) : ShadeStyle() {
 

orx-shade-styles/src/main/kotlin/NPointGradient.kt

@@ -0,0 +1,35 @@
+package org.openrndr.extra.shadestyles
+
+import org.openrndr.color.ColorRGBa
+import org.openrndr.draw.ShadeStyle
+import org.openrndr.extra.parameters.Description
+import org.openrndr.math.Vector2
+import org.openrndr.math.Vector3
+
+@Description("N-Point gradient")
+class NPointGradient(
+        numPoints: Int,
+        colors: Array<ColorRGBa>,
+        points: Array<Vector2> = arrayOf(Vector2.ZERO)) : ShadeStyle() {
+
+    var numPoints: Int by Parameter()
+    var colors: Array<ColorRGBa> by Parameter()
+    var points: Array<Vector2> by Parameter()
+
+    init {
+        this.numPoints = numPoints
+        this.colors = colors
+        this.points = points
+
+        fragmentTransform = """
+                float sum = 0;
+                vec4 rgba = vec4(0.0);
+                for(int i=0; i<p_numPoints; i++) {
+                    float dist = 1.0 / (1.0 + distance(p_points[i], c_screenPosition));
+                    sum += dist;
+                    rgba += p_colors[i] * dist;
+                }
+                x_fill = rgba/sum;
+        """
+    }
+}