Refactor of scroll and shaping

- Scroll is identified by line index and layout index, instead of just
  layout index
- Shaping has the option to prune, where caches outside of the scroll
  view are cleared
- Syntax editor no longer requires layout of all lines, only of lines
  inside scroll
- BufferLine has a metadata field that can be used by other abstractions
  to know when text was changed

examples/editor-orbclient/src/main.rs

@@ -88,7 +88,7 @@ fn main() {
     let mut mouse_y = -1;
     let mut mouse_left = false;
     loop {
-        editor.shape_as_needed();
+        editor.shape_as_needed(true);
         if editor.buffer().redraw() {
             let instant = Instant::now();