fix: window corner handling

src/core.rs

@@ -38,6 +38,7 @@ pub struct Window {
     pub show_close: bool,
     pub show_maximize: bool,
     pub show_minimize: bool,
+    pub is_maximized: bool,
     height: f32,
     width: f32,
 }
@@ -141,6 +142,7 @@ impl Default for Core {
                 show_maximize: true,
                 show_minimize: true,
                 show_window_menu: false,
+                is_maximized: false,
                 height: 0.,
                 width: 0.,
             },