Previously Alacritty was using two different ways to reference lines in
the terminal. Either a `usize`, or a `Line(usize)`. These indexing
systems both served different purposes, but made it difficult to reason
about logic involving these systems because of its inconsistency.

To resolve this issue, a single new `Line(i32)` type has been
introduced.  All existing references to lines and points now rely on
this definition of a line.

The indexing starts at the top of the terminal region with the line 0,
which matches the line 1 used by escape sequences. Each line in the
history becomes increasingly negative and the bottommost line is equal
to the number of visible lines minus one.

Having a system which goes into the negatives allows following the
escape sequence's indexing system closely, while at the same time making
it trivial to implement `Ord` for points.

The Alacritty UI crate is the only place which has a different indexing
system, since rendering and input puts the zero line at the top of the
viewport, rather than the top of the terminal region.

All instances which refer to a number of lines/columns instead of just a
single Line/Column have also been changed to use a `usize` instead. This
way a Line/Column will always refer to a specific place in the grid and
no confusion is created by having a count of lines as a possible index
into the grid storage.

Fixes #4597.

Co-authored-by: Christian Duerr <contact@christianduerr.com>

This changes the minimum terminal dimensions from 2 lines and 2 columns,
to 1 line and 2 columns.

This also reworks the `SizeInfo` to store the number of columns and
lines and consistently has only the terminal lines/columns stored,
instead of including the message bar and search in some places of the
Alacritty renderer/input.

These new changes also make it easy to properly start the selection
scrolling as soon as the mouse is over the message bar, instead of
waiting until it is beyond it.

Fixes #4207.

Co-authored-by: Christian Duerr <contact@christianduerr.com>

Co-authored-by: Christian Duerr <contact@christianduerr.com>

Fixes #4082.

Instead of just always falling back to the shell specified in the passwd
file when no config or cli shell was specified, Alacritty will not first
look at the `$SHELL` environment variable. If this is unset, it will
still read the passwd file.

Since macOS is a bit peculiar and does not set the `$SHELL` environment
variable by default, it is set manually to the shell used by Alacritty
while any existing `$SHELL` variables are ignored. This matches the
behavior of iTerm and Terminal.app.

Co-authored-by: Christian Duerr <contact@christianduerr.com>

This commit removes font dependency from alacritty_terminal,
so it'll simplify the usage of alacritty_terminal as a library,
since you won't link to system's libraries anymore. It also
moves many alacritty related config options from it.

Fixes #3393.

Keeping the license as part of every file bloats up the files
unnecessarily and introduces an additional overhead to the creation of
new modules.

Since cargo already provides excellent dependency management, most of
the code-reuse of Alacritty should occur through Rust's dependency
management instead of copying it source.

If code is copied partially, copying the license from the main license
file should be just as easy as copying from the top of the file and
making some adjustments based on where it is used is likely necessary
anyways.