[kit] Make a bunch of data structures public

MetaballsKit/Memory.swift

@@ -17,9 +17,25 @@ extension UnsafeMutableRawPointer {
 }
 
 /// 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
+public struct Float4 {
+    public var x: Float = 0
+    public var y: Float = 0
+    public var z: Float = 0
+    public var w: Float = 0
+
+    public init() { }
+
+    public init(_ x: Float, _ y: Float, _ z: Float, _ w: Float) {
+        self.x = x
+        self.y = y
+        self.z = z
+        self.w = w
+    }
+
+    public init(r: Float, g: Float, b: Float, a: Float) {
+        x = r
+        y = g
+        z = b
+        w = a
+    }
 }