Fix `Context::repaint_causes` returning no causes (#4248)

It would return the causes for repainting again collected this frame,
instead of the cause for repainting the current frame.

* Part of https://github.com/emilk/egui/issues/3931

crates/egui/src/context.rs

@@ -252,7 +252,7 @@ struct ViewportState {
 }
 
 /// What called [`Context::request_repaint`]?
-#[derive(Clone, Debug)]
+#[derive(Clone)]
 pub struct RepaintCause {
     /// What file had the call that requested the repaint?
     pub file: &'static str,
@@ -261,6 +261,12 @@ pub struct RepaintCause {
     pub line: u32,
 }
 
+impl std::fmt::Debug for RepaintCause {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        write!(f, "{}:{}", self.file, self.line)
+    }
+}
+
 impl RepaintCause {
     /// Capture the file and line number of the call site.
     #[allow(clippy::new_without_default)]
@@ -1465,7 +1471,7 @@ impl Context {
         self.read(|ctx| {
             ctx.viewports
                 .get(&ctx.viewport_id())
-                .map(|v| v.repaint.causes.clone())
+                .map(|v| v.repaint.prev_causes.clone())
         })
         .unwrap_or_default()
     }