[Metaballs] Sort out the Metal kernel declaration

Metaballs/Shaders.metal

@@ -9,4 +9,15 @@
 #include <metal_stdlib>
 using namespace metal;
 
+typedef struct {
+    float2 position;
+    float radius;
+} Ball;
 
+kernel void
+sampleFieldKernel(const device Ball* metaballs  [[buffer(0)]],
+                  device float* samples         [[buffer(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.
+}