Add shaders for marching squares

MetaballsKit/MarchingSquares.metal

@@ -0,0 +1,45 @@
+//
+//  MarchingSquares.metal
+//  Metaballs
+//
+//  Created by Eryn Wells on 10/14/18.
+//  Copyright © 2018 Eryn Wells. All rights reserved.
+//
+
+#include <metal_stdlib>
+#include "ShaderTypes.hh"
+using namespace metal;
+
+struct Rect {
+    float4x4 transform;
+    float4 color;
+};
+
+struct RasterizerData {
+    float4 position [[position]];
+    float4 color;
+    float2 textureCoordinate;
+};
+
+vertex RasterizerData
+gridVertexShader(constant Vertex *vertexes [[buffer(0)]],
+                 constant Rect *rects [[buffer(1)]],
+                 constant RenderParameters &renderParameters [[buffer(2)]],
+                 uint vid [[vertex_id]],
+                 uint instid [[instance_id]])
+{
+    Vertex v = vertexes[vid];
+
+    Rect rect = rects[instid];
+
+    RasterizerData out;
+    out.position = renderParameters.projection * rect.transform * float4(v.position.xy, 0, 1);
+    out.textureCoordinate = v.textureCoordinate;
+    return out;
+}
+
+fragment float4
+gridFragmentShader(RasterizerData in [[stage_in]])
+{
+    return in.color;
+}