Add badge to readme showing latest stable release

See merge request !1510

Fix some compiler warnings

See merge request STJr/SRB2!1504

The year is 2021

See merge request !1500

Some of these integers exceed the precision of float. In
that case the number is rounded. The rounding shouldn't
matter too much anyway, so just shut the compiler up.

2.2.9 prep

See merge request STJr/SRB2!1499

Stop using metatables with light userdata

See merge request STJr/SRB2!1495

Update URL on the addon screen

See merge request STJr/SRB2!1489

Fix MusicChange hook not returning some values correctly

See merge request STJr/SRB2!1496

See 7df6a309 and 83a87042. I didn't realize that light
userdata's metatable is shared--like numbers or strings.
So it cannot be paired with a metatable.

I also made a few minor tweaks to Lua cvars, other than
accounting for the double pointer in the userdata.

This reverts commit d4c08a84, reversing
changes made to e100f21d.

Attempt to resolve teleportation resync issue

See merge request STJr/SRB2!1490

Currently WIP: Blendmode and transparency support for MORE STUFF, including FOFs and linedefs (ported from Kart internal).

(I haven't been able to test this port due to requiring a patch.pk3 I do not have, and the Colour Cube settings are currently broken, but wanted to get the bulk of the work done while I had spare time and energy to later patch up.)

Last-minute fixes

See merge request STJr/SRB2!1483

This reverts commit c51c4787.

Give TOL_ERZ3 an appropriate identifier (closes #516)

Closes #516

See merge request STJr/SRB2!1471

Let Lua cvars use preset values in addition to MIN and MAX

See merge request STJr/SRB2!1480

Unset zstring if a preset value is used

Closes #487

See merge request STJr/SRB2!1479

This is only applicable for bounded cvars (MIN, MAX),
since otherwise there's no way to allocate a zstring.

Cvars could now have a range (MIN, MAX) plus some preset
values, but Lua could not take advantage of this due to
table order not being guaranteed.

Fix a dedicated crash in Special Stage intermissions (2.2.9 RC1)

See merge request STJr/SRB2!1475

Spritedef add hotfix

See merge request STJr/SRB2!1476

Possible netgame quit crash hotfix

Closes #487

See merge request STJr/SRB2!1477