Constrain demo windows to the space left after the panels (#7785)

crates/egui_demo_lib/src/demo/mod.rs

@@ -61,5 +61,5 @@ pub trait Demo {
     fn name(&self) -> &'static str;
 
     /// Show windows, etc
-    fn show(&mut self, ctx: &egui::Context, open: &mut bool);
+    fn show(&mut self, ui: &mut egui::Ui, open: &mut bool);
 }