Add `Context::current_pass_index` (#7276)

This can be used by developers who wants to add diagnostics when there
is a multi-pass egui frame.

crates/egui/src/context.rs

@@ -1539,6 +1539,9 @@ impl Context {
     /// The total number of completed passes (usually there is one pass per rendered frame).
     ///
     /// Starts at zero, and is incremented for each completed pass inside of [`Self::run`] (usually once).
+    ///
+    /// If you instead want to know which pass index this is within the current frame,
+    /// use [`Self::current_pass_index`].
     pub fn cumulative_pass_nr(&self) -> u64 {
         self.cumulative_pass_nr_for(self.viewport_id())
     }
@@ -1554,6 +1557,18 @@ impl Context {
         })
     }
 
+    /// The index of the current pass in the current frame, starting at zero.
+    ///
+    /// Usually this is zero, but if something called [`Self::request_discard`] to do multi-pass layout,
+    /// then this will be incremented for each pass.
+    ///
+    /// This just reads the value of [`PlatformOutput::num_completed_passes`].
+    ///
+    /// To know the total number of passes ever completed, use [`Self::cumulative_pass_nr`].
+    pub fn current_pass_index(&self) -> usize {
+        self.output(|o| o.num_completed_passes)
+    }
+
     /// Call this if there is need to repaint the UI, i.e. if you are showing an animation.
     ///
     /// If this is called at least once in a frame, then there will be another frame right after this.
@@ -1726,7 +1741,7 @@ impl Context {
     /// This means the first pass will look glitchy, and ideally should not be shown to the user.
     /// So [`crate::Grid`] calls [`Self::request_discard`] to cover up this glitches.
     ///
-    /// There is a limit to how many passes egui will perform, set by [`Options::max_passes`].
+    /// There is a limit to how many passes egui will perform, set by [`Options::max_passes`] (default=2).
     /// Therefore, the request might be declined.
     ///
     /// You can check if the current pass will be discarded with [`Self::will_discard`].