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.

tests/test_viewports/src/main.rs

@@ -478,7 +478,7 @@ fn drop_target<R>(
 
     ui.painter().set(
         background_id,
-        egui::epaint::RectShape::new(rect, style.rounding, fill, stroke),
+        egui::epaint::RectShape::new(rect, style.rounding, fill, stroke, egui::StrokeKind::Inside),
     );
 
     egui::InnerResponse::new(ret, response)