Add multithreading option for CMake

See merge request STJr/SRB2!1204

Fix steep slope jank

See merge request KartKrew/Kart!318

The big large slope physics

See merge request KartKrew/Kart!320

Fixes respawning over slopes after crossing a drop.

This should fix the janky falling off steep slopes and let you cling to them.
Because when falling off a sector the edge of the player must not intersect
the edge of the higher sector, it is trickier than step up. The height
difference at the line must be stored to be useful.

Fix gzip failing to compress SRB2's objdump on non-Windows.

See merge request STJr/SRB2!1191

This is for slopes. When moving across a slope the height changes. Therefore
compare the height at the edge where slopes meet so only a wall will block the
player, and not a very steep slope.

2.2.8 prep

See merge request STJr/SRB2!1170

Fix uninitialized variable issue in patches with missing grAb chunks

See merge request STJr/SRB2!1171

Prevent Canarivore gas from teleporting up walls

See merge request STJr/SRB2!1154

Fix #178

Closes #178

See merge request STJr/SRB2!1112

Remove "The OpenGL renderer is incomplete" notice (closes #281)

Closes #281

See merge request STJr/SRB2!1169

Fix graphics replacements not taking effect via -file

See merge request STJr/SRB2!1168

This reverts commit 3883d44b.

And one line from 5c1b3baf.

This is needed for graphics replacements to take effect.