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-test/src/main.rs

@@ -16,7 +16,7 @@ fn redraw(
     let bg_color = orbclient::Color::rgb(0x34, 0x34, 0x34);
     let font_color = Color::rgb(0xFF, 0xFF, 0xFF);
 
-    editor.shape_as_needed();
+    editor.shape_as_needed(true);
     if editor.buffer().redraw() {
         let instant = Instant::now();