Require a StrokeKind when painting rectangles with strokes (#5648)

This is a breaking change, requiring users to think about wether the
stroke is inside/centered/outside the rect.

When in doubt, add `egui::StrokeKind::Inside` to the function call.

crates/egui_demo_lib/src/demo/misc_demo_window.rs

@@ -390,6 +390,7 @@ impl BoxPainting {
                     self.rounding,
                     ui.visuals().text_color().gamma_multiply(0.5),
                     Stroke::new(self.stroke_width, Color32::WHITE),
+                    egui::StrokeKind::Inside,
                 );
             }
         });