Render header blocks as horizontall fixed (like the gutter)

Tweak editor layout to perform horizontal autoscroll before
laying out the blocks, so that they can access the scroll position.

crates/editor/src/display_map/block_map.rs

@@ -69,6 +69,7 @@ where
 pub struct BlockContext<'a> {
     pub cx: &'a AppContext,
     pub anchor_x: f32,
+    pub scroll_x: f32,
     pub gutter_width: f32,
     pub gutter_padding: f32,
     pub em_width: f32,
@@ -951,6 +952,7 @@ mod tests {
                             cx,
                             anchor_x: 0.,
                             gutter_padding: 0.,
+                            scroll_x: 0.,
                             gutter_width: 0.,
                             line_height: 0.,
                             em_width: 0.,