[MetaballsKit] Add a metal file to the framework

MetaballsKit/SamplingKernel.metal

@@ -0,0 +1,24 @@
+//
+//  SamplingKernel.metal
+//  Metaballs
+//
+//  Created by Eryn Wells on 8/1/17.
+//  Copyright © 2017 Eryn Wells. All rights reserved.
+//
+
+#include <metal_stdlib>
+using namespace metal;
+
+typedef struct {
+    float radius;
+    float2 position;
+    float2 velocity;
+} Ball;
+
+kernel void
+sampleFieldKernel(const device Ball* metaballs      [[buffer(0)]],
+                  texture2d<half, access::write>    [[texture(1)]],
+                  uint2 gid                         [[thread_position_in_grid]])
+{
+    // TODO: Compute a sample for this pixel given the field data, and write it to the out texture.
+}