ref(container): split size and position calculations

examples/demo.zig

@@ -92,11 +92,11 @@ pub fn main() !void {
             .color = .light_blue,
             .sides = .all,
         },
-        .fixed_size = .{ .x = 200 },
+        .size = .{ .x = 200 },
     }, .{}));
     try container.append(try App.Container.init(allocator, .{
         .rectangle = .{ .fill = .blue },
-        .fixed_size = .{ .x = 30 },
+        .size = .{ .x = 30 },
     }, .{}));
     defer container.deinit(); // also de-initializes the children
 
@@ -146,8 +146,7 @@ pub fn main() !void {
         }
 
         try renderer.resize();
-        container.reposition(.{});
-        container.resize(renderer.size);
+        container.resize(.{}, renderer.size);
         try renderer.render(@TypeOf(container), &container);
         try renderer.flush();
     }