good but wrong teardown order; device wait idle

src/nu/Render.zig

@@ -30,11 +30,33 @@ pub const Options = struct {
 var sc: au.SwapChain = undefined;
 var flights: au.Flights = undefined;
 
+const PresentCallback = *const fn (au.CommandBufferProxy) void;
+var present_callbacks: std.ArrayList(PresentCallback) = undefined;
+
+pub fn add_present_callback(cb: PresentCallback) !void {
+    if (std.mem.indexOfScalar(PresentCallback, present_callbacks.items, cb)) |_| {
+        return;
+    } else {
+        try present_callbacks.append(cb);
+    }
+}
+
+pub fn remove_present_callback(cb: PresentCallback) void {
+    if (std.mem.indexOfScalar(PresentCallback, present_callbacks.items, cb)) |idx| {
+        _ = present_callbacks.orderedRemove(idx);
+    } else {
+        return;
+    }
+}
+
 pub fn init(alloc: std.mem.Allocator) !void {
     // todo pick apart au into helpers; not a sub-module filled with its own globals.
     try au.init(alloc);
     errdefer au.deinit();
 
+    present_callbacks = std.ArrayList(PresentCallback).init(alloc);
+    errdefer present_callbacks.deinit();
+
     sc = try au.SwapChain.init(alloc);
     errdefer sc.deinit();
 
@@ -67,6 +89,9 @@ pub fn frame() !void {
 
         try cmd.beginCommandBuffer(&.{ .flags = .{ .one_time_submit_bit = true } });
         target.begin_rendering(cmd, render_area);
+        for (present_callbacks.items) |cb| {
+            cb(cmd);
+        }
         target.end_rendering(cmd);
         try cmd.endCommandBuffer();
 
@@ -98,6 +123,7 @@ pub fn frame() !void {
 }
 
 pub fn deinit() void {
+    present_callbacks.deinit();
     au.D.deviceWaitIdle() catch {};
     flights.deinit();
     sc.deinit();