metal: retain IOSurfaceLayer ourselves instead of relying on the view

If this was Swift code, we'd be using a strong reference, which would
retain the layer for us and release it when the object is deallocated,
but this is Zig land so we have to do that manually.

NOTE: We don't *have* to do this, but it fits much better with Zig idiom
and hopefully avoids potential future footguns. We should do this to any
autoreleased objects that we persist a reference to in a Zig struct.

src/renderer/Metal.zig

@@ -165,9 +165,7 @@ pub fn init(alloc: Allocator, opts: rendererpkg.Options) !Metal {
 pub fn deinit(self: *Metal) void {
     self.queue.release();
     self.device.release();
-
-    // NOTE: We don't release the layer here because that should be taken
-    //       care of automatically when the hosting view is destroyed.
+    self.layer.release();
 }
 
 pub fn loopEnter(self: *Metal) void {

src/renderer/metal/IOSurfaceLayer.zig

@@ -21,11 +21,14 @@ var Subclass: ?objc.Class = null;
 layer: objc.Object,
 
 pub fn init() !IOSurfaceLayer {
+    // The layer returned by `[CALayer layer]` is autoreleased, which means
+    // that at the end of the current autorelease pool it will be deallocated
+    // if it isn't retained, so we retain it here manually an extra time.
     const layer = (try getSubclass()).msgSend(
         objc.Object,
         objc.sel("layer"),
         .{},
-    );
+    ).retain();
     errdefer layer.release();
 
     // The layer gravity is set to top-left so that the contents aren't