Fix CentralPanel::show_inside_dyn to round panel_rect (#7868)

While using CentralPanel::show_inside_dyn inside egui_tiles I've noticed
that sometimes it renders "Unaligned" message with certain tile
positions. Match SidePanel behavior by calling round_ui() on the panel
rect.
<img width="364" height="173" alt="image"
src="https://github.com/user-attachments/assets/08a9cef3-67f8-49af-9ea7-75aff711186a"
/>

---------

Co-authored-by: Emil Ernerfeldt <emil.ernerfeldt@gmail.com>

crates/egui/src/containers/panel.rs

@@ -1066,7 +1066,7 @@ impl CentralPanel {
             id,
             UiBuilder::new()
                 .layer_id(LayerId::background())
-                .max_rect(ctx.available_rect().round_ui()),
+                .max_rect(ctx.available_rect()),
         );
         panel_ui.set_clip_rect(ctx.content_rect());
 

crates/egui/src/context.rs

@@ -798,7 +798,7 @@ impl Context {
                 Id::new((ctx.viewport_id(), "__top_ui")),
                 UiBuilder::new()
                     .layer_id(LayerId::background())
-                    .max_rect(ctx.available_rect().round_ui()),
+                    .max_rect(ctx.available_rect()),
             );
 
             {

crates/egui/src/placer.rs

@@ -1,4 +1,5 @@
 use crate::{Layout, Painter, Pos2, Rect, Region, Vec2, grid, vec2};
+use emath::GuiRounding as _;
 
 #[cfg(debug_assertions)]
 use crate::{Align2, Color32, Stroke};
@@ -92,6 +93,7 @@ impl Placer {
         } else {
             self.layout.available_rect_before_wrap(&self.region)
         }
+        .round_ui()
     }
 
     /// Amount of space available for a widget.

crates/egui_demo_lib/tests/snapshots/demos/Misc Demos.png

@@ -1,3 +1,3 @@
 version https://git-lfs.github.com/spec/v1
-oid sha256:604f716e687fc26abba92769fe2dae75d850b18598d2e8a9524451ab0f760251
-size 65403
+oid sha256:56b44d26946770c0878e11e3197633697ad339a7e8fcffe7279a6b4c45cd3582
+size 65384