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

src/font/system.rs

@@ -177,7 +177,7 @@ impl FontSystem {
         }
 
         #[cfg(not(target_arch = "wasm32"))]
-        log::info!(
+        log::debug!(
             "Parsed {} font faces in {}ms.",
             db.len(),
             now.elapsed().as_millis()