Fix Ctrl+Backspace in the console causing a crash

See merge request STJr/SRB2!2437

Fix Linedef type 466

Closes #1246

See merge request STJr/SRB2!2453

Fix #1222

Closes #1222

See merge request STJr/SRB2!2448

Fix composite "flats as patch" name overlap crash

See merge request STJr/SRB2!2450

Fix keyboard input when certain keys overlap with dead keys

Closes #1174

See merge request STJr/SRB2!2443

Like with ccache, we want to make sure the git checkout is considered
clean during build.

Placing these directories at the root will make the git revision
detection consider the working directory to be dirty.

Fixes: KartKrew/Kart#1405

The "clone" strategy clones the entire repository, which takes
a significant amount of time. It's not needed, because the default
"fetch" strategy will reuse the executor's local clone and clean it
before the job scripts run.

There's no need to let these artifacts pile up since the only ones that
will be relevant to non-programmers are the latest from any given ref.

Ring Racers' object file output is considerably larger than SRB2.
50M is insufficient to hold even a single build.

The singular .gitlab-ci.yml is pretty beefy. Splitting it into
smaller files by job and job template will make maintaining the
individual jobs easier.

Fix backwards compat for SPR2 Super (resolves #1225)

Closes #1225

See merge request STJr/SRB2!2410

Fix buffer overflow when invoking MusicChange hook

See merge request STJr/SRB2!2441

Also fix Ctrl+C / Ctrl+X emptying the clipboard without a text selection