Improve rendering performance

This PR combines a couple of optimizations to drastically reduce the
time it takes to gather everything necessary for rendering Alacritty's
terminal grid.

To help with the iteration over the grid, the `DisplayIter` which made
heavy use of dynamic dispatch has been replaced with a simple addition
to the `GridIterator` which also had the benefit of making the code a
little easier to understand.

The hints/search check for each cell was always performing an array
lookup before figuring out that the cell is not part of a hint or
search. Since the general case is that the cell is neither part of hints
or search, they've been wrapped in an `Option` to make verifying their
activity a simple `is_some()` check.

For some reason the compiler was also struggling with the `cursor`
method of the `RenderableContent`. Since the iterator is explicitly
drained, the performance took a hit of multiple milliseconds for a
single branch. Our implementation does never reach the case where
draining the iterator would be necessary, so this sanity check has just
been replaced with a `debug_assert`.

Overall this has managed to reduce the time it takes to collect all
renderable content from ~7-8ms in my large grid test to just ~3-4ms.

1 files changed, 2 insertions, 2 deletions

diff --git a/alacritty_terminal/src/term/mod.rs b/alacritty_terminal/src/term/mod.rs
index 409d4ebe..2fb4da34 100644
--- a/alacritty_terminal/src/term/mod.rs
+++ b/alacritty_terminal/src/term/mod.rs
@@ -15,7 +15,7 @@ use crate::ansi::{
 };
 use crate::config::Config;
 use crate::event::{Event, EventListener};
-use crate::grid::{Dimensions, DisplayIter, Grid, Scroll};
+use crate::grid::{Dimensions, Grid, GridIterator, Scroll};
 use crate::index::{self, Boundary, Column, Direction, Line, Point, Side};
 use crate::selection::{Selection, SelectionRange};
 use crate::term::cell::{Cell, Flags, LineLength};
@@ -1828,7 +1828,7 @@ impl RenderableCursor {
 ///
 /// This contains all content required to render the current terminal view.
 pub struct RenderableContent<'a> {
-    pub display_iter: DisplayIter<'a, Cell>,
+    pub display_iter: GridIterator<'a, Cell>,
     pub selection: Option<SelectionRange>,
     pub cursor: RenderableCursor,
     pub display_offset: usize,