add first pass metal shader

src/shaders/cell.metal

@@ -0,0 +1,35 @@
+vertex float4 basic_vertex(unsigned int vid [[ vertex_id ]]) {
+  // Where we are in the grid (x, y) where top-left is origin
+  float2 grid_coord = float2(0.0f, 0.0f);
+
+  // The size of a single cell in pixels
+  float2 cell_size = float2(75.0f, 100.0f);
+
+  // Convert the grid x,y into world space x, y by accounting for cell size
+  float2 cell_pos = cell_size * grid_coord;
+
+  // Turn the cell position into a vertex point depending on the
+  // vertex ID. Since we use instanced drawing, we have 4 vertices
+  // for each corner of the cell. We can use vertex ID to determine
+  // which one we're looking at. Using this, we can use 1 or 0 to keep
+  // or discard the value for the vertex.
+  //
+  // 0 = top-right
+  // 1 = bot-right
+  // 2 = bot-left
+  // 3 = top-left
+  float2 position;
+  position.x = (vid == 0 || vid == 1) ? 1.0f : 0.0f;
+  position.y = (vid == 0 || vid == 3) ? 0.0f : 1.0f;
+
+  // Calculate the final position of our cell in world space.
+  // We have to add our cell size since our vertices are offset
+  // one cell up and to the left. (Do the math to verify yourself)
+  cell_pos = cell_pos + cell_size * position;
+
+  return float4(cell_pos.x, cell_pos.y, 0.5f, 1.0f);
+}
+
+fragment half4 basic_fragment() {
+  return half4(1.0);
+}