fix(container/border): correct location and rendering of separators between child elements

examples/container.zig

@@ -28,10 +28,9 @@ pub fn main() !void {
             .color = .blue,
             .corners = .rounded,
             .sides = .all(),
-            .separator = .{ .enabled = false },
+            .separator = .{ .enabled = true },
         },
         .layout = .{
-            .gap = 2,
             .padding = .all(5),
             .direction = .vertical,
         },