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

* 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).

Give the Spectator Eggrobos the ability to move left and right relative to their angle, with initial direction depending on MTF_OBJECTSPECIAL/MTF_AMBUSH flag presence. (May need more tweaking before putting in CEZ3)

Motor's new Eggman sprites, along with some other sweet bits of boss polish. What else can I say? Just play it.

Weapon ammo penalty indicator

See merge request STJr/SRB2Internal!297

Autobrake polish

Closes #176

See merge request STJr/SRB2Internal!293

Ported over from Kart

P_PlayerCanDamage fix

See merge request STJr/SRB2Internal!291

# Conflicts:
#	src/d_netfil.c