add(container/layout): sizing:fixed option

examples/container.zig

@@ -31,8 +31,12 @@ pub fn main() !void {
             .separator = .{ .enabled = false },
         },
         .layout = .{
-            .gap = 3,
-            .padding = .all(2),
+            // .gap = 1,
+            .sizing = .{
+                .width = .{ .fixed = 100 },
+                .height = .{ .fixed = 50 },
+            },
+            .padding = .all(5),
             .direction = .horizontal,
         },
     });