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.
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