[app,fw] Pass color information into the shader

MetaballsKit/Memory.swift

@@ -15,3 +15,11 @@ extension UnsafeMutableRawPointer {
         return advanced(by: stride)
     }
 }
+
+/// Metal's float4 type. 4 bytes per float, 16 bytes total, 16 byte aligned.
+struct Float4 {
+    var x: Float = 0
+    var y: Float = 0
+    var z: Float = 0
+    var w: Float = 0
+}