Panel: never overflow available width, nor max_width (#8198)

* Closes https://github.com/emilk/egui/issues/8055
* Closes https://github.com/emilk/egui/pull/8056

This changes the behavior of `Panel` to NEVER overflow
`Panel::max_size`, nor the available space in the parent UI.

If you do overflow it, the content will be silently clipped.

crates/egui_kittest/tests/tests.rs

@@ -1,3 +1,6 @@
+#![cfg(feature = "snapshot")]
+#![cfg(feature = "wgpu")]
+
 use egui::{Modifiers, ScrollArea, Vec2, include_image};
 use egui_kittest::{Harness, SnapshotResults};
 use kittest::Queryable as _;
@@ -122,6 +125,7 @@ fn test_scroll_harness() -> Harness<'static, bool> {
         )
 }
 
+#[cfg(feature = "snapshot")]
 #[test]
 fn test_scroll_to_me() {
     let mut harness = test_scroll_harness();