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/terminal/src/main.rs

@@ -39,7 +39,7 @@ fn main() {
     buffer.set_text(&text, attrs, Shaping::Advanced);
 
     // Perform shaping as desired
-    buffer.shape_until_scroll();
+    buffer.shape_until_scroll(true);
 
     // Default text color (0xFF, 0xFF, 0xFF is white)
     const TEXT_COLOR: Color = Color::rgb(0xFF, 0xFF, 0xFF);