Add example of using list clipper API

Somewhat relates to #438

imgui-examples/examples/long_list.rs

@@ -0,0 +1,33 @@
+/// Demonstrates using the "list clipper" to efficiently display long
+/// lists in a scrolling area.
+///
+/// You specify the height per item, and the `ListClipper` API will
+/// provide which item index are visible. This avoids having to create
+/// thousands of items only for them to not be made visible.
+///
+/// Note this requires a fixed (or easily computable) height per item.
+use imgui::*;
+
+mod support;
+
+fn main() {
+    let lots_of_words: Vec<String> = (0..10000)
+        .map(|x| format!("Line {}", x).into())
+        .collect();
+
+    let system = support::init(file!());
+    system.main_loop(move |_, ui| {
+        Window::new(im_str!("Hello long world"))
+            .size([300.0, 110.0], Condition::FirstUseEver)
+            .build(ui, || {
+                let mut clipper = imgui::ListClipper::new(lots_of_words.len() as i32)
+                    .items_height(ui.current_font_size())
+                    .begin(ui);
+                while clipper.step() {
+                    for row_num in clipper.display_start()..clipper.display_end() {
+                        ui.text(&lots_of_words[row_num as usize]);
+                    }
+                }
+            });
+    });
+}