This resolves a rounding issue when converting to the 26.6 format used
by freetype for character sizes.

This rounding behavior is taken from cairo:
https://gitlab.freedesktop.org/cairo/cairo/-/blob/master/src/cairo-ft-font.c#L900-903

There are various different implementations of the F26Dot6 conversion
online, but the rounding that cairo does seems to be the most common.
Since cairo is very commonly used, it should produce good results
compared with the rest of Linux text rendering.

Fixes #2780.

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

In addition it also starts respecting Fontconfig's `autohint`
and `hinting` options.

Fixes #3534.

Fixes #3915.

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.

Fixes #1754.

Since there were some problems with clippy suggesting changes that were
not yet available in the oldest supported Rust compiler of Alacritty,
the clippy stage has been moved from stable to 1.37.0.