Horizontal scrolling (#663)

* First pass (missing rendering the bar)

* Render horizontal bars, and change Window scroll API

* emath: add impl Index + IndexMut for Align2

* Scrolling: fix subtle sizing bugs

* Add horizontal scrolling to color test

* try to wrap content before showing scrollbars, + add auto-shrink option

* Add hscroll to the misc demo window

* Fix for putting wrapping labels in an infinitely wide layout

* Add a egui_asserts to protect against nans in the layout engine

* Add line about horizontal scrolling to changelog

* Add example to docs of ScrollArea

* code cleanup

egui_demo_lib/src/backend_panel.rs

@@ -323,14 +323,14 @@ impl EguiWindows {
 
         egui::Window::new("🔧 Settings")
             .open(settings)
-            .scroll(true)
+            .vscroll(true)
             .show(ctx, |ui| {
                 ctx.settings_ui(ui);
             });
 
         egui::Window::new("🔍 Inspection")
             .open(inspection)
-            .scroll(true)
+            .vscroll(true)
             .show(ctx, |ui| {
                 ctx.inspection_ui(ui);
             });