Alacritty should avoid rendering cells as early as possible

[chrisduerr]

Currently even with a majority of the grid empty, Alacritty will go through the creation of renderable cells for every single cell visible and only filter out empty cells after this transformation is done. As a result this significantly slows down Alacritty's rendering with extremely dense grids that are largely empty.

Ideally instead of iterating over empty cells and applying color transforms to them, these should not even be yielded by the display iterator. Currently the only major problem with that is the presence of things like selection which require empty cells to be yielded so they can be transformed into the selection.