WIP: container rendering for borders + container element rendering

examples/container.zig

@@ -23,6 +23,7 @@ pub fn main() !void {
     defer renderer.deinit();
 
     var container = try App.Container.init(allocator, .{});
+    try container.append(try App.Container.init(allocator, .{}));
     defer container.deinit();
 
     // NOTE: should the min-size here be required?