Fix an issue where completing a stage normally would not correctly set startrings for the next one (as opposed to warping directly or dying inside it).

This might LOOK like an unrelated change, but it needs a new player.dta, so might as well keep that all localised!

* https://cdn.discordapp.com/attachments/428262628893261828/621129794045870108/srb20077.gif
* https://cdn.discordapp.com/attachments/428262628893261828/621129785124585502/srb20078.gif
* I will be accepting no further questions at this time.

These have been in srb2.pk3 for a while, but they were inaccessible because of not having in-game references.

Lightning Polys

See merge request STJr/SRB2Internal!294

Linedef type 461 (Spawn Object)

See merge request STJr/SRB2Internal!304

Various improvements and changes to multiplayer menu

See merge request STJr/SRB2Internal!300

NiGHTS attack fixes

Closes #205

See merge request STJr/SRB2Internal!319

Endings: Fireworks fallback graphics.

See merge request STJr/SRB2Internal!296

Flippost

Closes #206 and #191

See merge request STJr/SRB2Internal!320

Change flipped starpostz to define the top of the player object, for cases where scale on contact with starpost is different to spawn scale.

Newegg dot blag

See merge request STJr/SRB2Internal!305

Notably, can no longer use P_PlayJingle because of pre-fade.

* Store gravflip (resolves #206) and destscale in starposts. (Using the same field, taking advantage of the fact that object scale will always be positive!)
* Update the function signature of P_MixUp to accomodate both it and drawangle instead of doing it outside of the function.
* If the player is spawning from the start of the stage and it's from the ceiling, be in fall frames as requested (resolves #191).

Minor musicplus-related fixes

See merge request STJr/SRB2Internal!310

Ignore bots when looking for a player

Closes #190

See merge request STJr/SRB2Internal!301

Fix -skipintro in OpenGL

See merge request STJr/SRB2Internal!308

DEVMODE: 2D toggle command

See merge request STJr/SRB2Internal!309

* Comment out the notices that the two types of music are disabled (see issue #179; not a perfect fix, but good enough for now).

* Fix jingle captions dying immediately (caused by an S_StopMusic call in P_PlayJingle; resolved by modifying order of operations).

* Don't allow the stage to be reloaded in G_DoReborn.
* If you do ANY spawn after the very beginning moment, you're forced to be a spectator.
* Have the "%d player%s remaining" Co-op exiting count visible at the same time as spectator controls.

https://cdn.discordapp.com/attachments/428262628893261828/618812279127015475/srb20069.gif

* Genesis-style love and attention to the death event.
	* Only visibly decrement lives/rings when you're respawning (or game over, see below).
	* Faster no-button-press respawn.
* Game Over specific love.
	* Animation of Level Title font coming in from the sides.
	* https://cdn.discordapp.com/attachments/428262628893261828/617692325438554132/srb20067.gif
	* Change gameovertics to 10 seconds instead of 15.
	* Make the minimum time before you can force going to the Continue screen longer.
* Accomodate death in MP special stages as a form of exit.
	* Don't have your rings or spheres reset when you die in a special stage, so that the stage isn't softlocked with the new harder limits.
* Fix a bug with CoopLives_OnChange where changing to infinite lives didn't force a game-overed player to respawn.

Also, two not-quite death things which nonetheless were relevant to change:

* Fix quitting a special stage having some of the shared spheres/rings disappear into the aether.
* Fix a warning during compilation for the Ring Penalty print.