Refactor cell selection out of renderer

The terminal now has a `renderable_cells()` function that returns a
`RenderableCellIter` iterator. This allows reuse of the cell selection
code by multiple renderers, makes it testable, and makes it
independently optimizable.

The render API now takes an `Iterator<Item=IndexedCell>` to support both
the new renderable cells iterator and the `render_string()` method which
generates its own iterator.

The `vim_large_window_scoll` ref test was added here because it provides
a nice large and busy grid to benchmark the cell selection with.

1 files changed, 2 insertions, 1 deletions

diff --git a/tests/ref.rs b/tests/ref.rs
index 349b565e..a92da05f 100644
--- a/tests/ref.rs
+++ b/tests/ref.rs
@@ -78,6 +78,7 @@ mod reference {
         ll,
         vim_simple_edit,
         tmux_htop,
-        tmux_git_log
+        tmux_git_log,
+        vim_large_window_scroll
     }
 }