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, 0 insertions, 5 deletions

diff --git a/alacritty/src/input.rs b/alacritty/src/input.rs
index 3559b85e..948ec67f 100644
--- a/alacritty/src/input.rs
+++ b/alacritty/src/input.rs
@@ -996,7 +996,6 @@ mod tests {
     use glutin::event::{Event as GlutinEvent, VirtualKeyCode, WindowEvent};
 
     use alacritty_terminal::event::Event as TerminalEvent;
-    use alacritty_terminal::selection::Selection;
 
     use crate::config::Binding;
     use crate::message_bar::MessageBuffer;
@@ -1008,7 +1007,6 @@ mod tests {
 
     struct ActionContext<'a, T> {
         pub terminal: &'a mut Term<T>,
-        pub selection: &'a mut Option<Selection>,
         pub size_info: &'a SizeInfo,
         pub mouse: &'a mut Mouse,
         pub clipboard: &'a mut Clipboard,
@@ -1145,13 +1143,10 @@ mod tests {
                     ..Mouse::default()
                 };
 
-                let mut selection = None;
-
                 let mut message_buffer = MessageBuffer::new();
 
                 let context = ActionContext {
                     terminal: &mut terminal,
-                    selection: &mut selection,
                     mouse: &mut mouse,
                     size_info: &size,
                     clipboard: &mut clipboard,