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, 9 insertions, 4 deletions

diff --git a/alacritty_terminal/src/selection.rs b/alacritty_terminal/src/selection.rs
index 428b3f0e..47a49910 100644
--- a/alacritty_terminal/src/selection.rs
+++ b/alacritty_terminal/src/selection.rs
@@ -13,7 +13,7 @@ use crate::ansi::CursorShape;
 use crate::grid::{Dimensions, GridCell, Indexed};
 use crate::index::{Boundary, Column, Line, Point, Side};
 use crate::term::cell::{Cell, Flags};
-use crate::term::{RenderableCursor, Term};
+use crate::term::Term;
 
 /// A Point and side within that point.
 #[derive(Debug, Copy, Clone, PartialEq)]
@@ -56,10 +56,15 @@ impl SelectionRange {
     }
 
     /// Check if the cell at a point is part of the selection.
-    pub fn contains_cell(&self, indexed: &Indexed<&Cell>, cursor: RenderableCursor) -> bool {
+    pub fn contains_cell(
+        &self,
+        indexed: &Indexed<&Cell>,
+        point: Point,
+        shape: CursorShape,
+    ) -> bool {
         // Do not invert block cursor at selection boundaries.
-        if cursor.shape == CursorShape::Block
-            && cursor.point == indexed.point
+        if shape == CursorShape::Block
+            && point == indexed.point
             && (self.start == indexed.point
                 || self.end == indexed.point
                 || (self.is_block