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

emath/src/align.rs

@@ -172,6 +172,22 @@ impl Align2 {
     }
 }
 
+impl std::ops::Index<usize> for Align2 {
+    type Output = Align;
+
+    #[inline(always)]
+    fn index(&self, index: usize) -> &Align {
+        &self.0[index]
+    }
+}
+
+impl std::ops::IndexMut<usize> for Align2 {
+    #[inline(always)]
+    fn index_mut(&mut self, index: usize) -> &mut Align {
+        &mut self.0[index]
+    }
+}
+
 pub fn center_size_in_rect(size: Vec2, frame: Rect) -> Rect {
     Align2::CENTER_CENTER.align_size_within_rect(size, frame)
 }