Skybox hotfix

This branch fixes how, for a skybox with both a centerpoint and viewpoint, the first person view code did not take the centerpoint's angle into account. So when the centerpoint's angle is NOT 0 (or a multiple of 360), this results in the skybox "moving the wrong way" when you're viewing the skybox this way and moving about.

...yeah sorry guys, I commited to a copy of next again, whoops. Cherry-pick the relevant commit into master if this works out fine - after all, this shouldn't affect netgames I think (it's purely about rendering after all)

See merge request !56

Fix how FF_CUTEXTRA FOFs carry over cutting ability to FOFs below them in the same sector, even if they shouldn't do that

Always wondered why a flag meant for FOF physics was used in the rendering code, lol

* add skidtime, which we forgot before 2.1 release apparently
* change tics from INT16 to INT32
* change eflags from UINT8 to UINT16
* change actionspd/mindash/maxdash from INT32 to fixed_t

Splitscreen fix: half of GFZ1's invinc monitor should no longer appear above the bridge for player 2

I don't know if there's any other vid.height/viewheight confusion like this around, but that was the cause apparently

initialize flipcam2
player 2 gets game over music now.
restore the other player's music when the dead player is done sulking about their game over

Fixed mashing buttons during fades causing crashes,
messed up behavior, record attack anywhere,
all the damn stupid bugs that it caused, basically

Until we use something besides Native MIDI to play
back MIDI music, MIDI volume changing is disabled
since it causes way too much of a damn headache.
(It's not even our fault, it's fucking MS.)

Apparently bosses and players used to use options >> 5 instead of 4 for z heights, or so it seems

Fix for teetering on PolyObjects

So... somebody goofed and didn't realise PolyObject sectors could be added to the sector node list for each object (which is referenced by mobj->touching_sectorlist), via their linedefs if they are nearby the player (yes, PolyObject linedefs are special and get to move about the level). As it turns out, this allows even INTANGIBLE PolyObjects to make you teeter in a seemingly inexplicable way.

What is happening is that PolyObject sectors, when they are added to the mentioned lists, are then checked under normal sector teetering conditions - if the player is above the floorheight by 24 FUs, you're officially teetering unless stated otherwise. The actual PolyObject teetering code can't help you here if the conditions are right, especially if they're taller than 24 FU in height.

There are a number of things wrong with the teetering code in general that I'd like to sort eventually, but at least now teetering on PolyObjects is fixed at last ...right? Please check the branch out if you can to check this, obviously.

See merge request !54

Fixes for patch scaling and V_FLIP usage

The following issues are fixed by this branch:
* a patch using both scaling and V_FLIP does not appear in the "correct" place on the screen. Thanks to LJSonic for pointing this out to me on 'fun
* Scaled and/or V_FLIPed patches wrap at the left/right screen edges even though they're not supposed to. V_FLIP patches at these edges may also crop the wrong side of the patch if they're at the right edge.

See merge request !60

Some fixes for portals

Specifically the following things are fixed in this branch:

* a memory leak resulting from not clearing away clipping-related arrays each tic you view a portal
* a very specific crash to do with portals sometimes (unintentionally) using a hack on drawsegs that don't actually belong to them ...which results in a crash if the drawsegs in question have midtextures. I reported it on the MB a year ago, with a test map included there: https://mb.srb2.org/showthread.php?t=38199&page=4#79
* another specific crash to do with mirrored (horizontally flipped) sprites that are scaled, particularly when you cover up the left edge of them via portals at the least. Needs more testing to be absolutely sure this is fixed, and is also reproducable in the test map linked in the post above

May be fine to merge changes into master too, I don't really know exactly

See merge request !42

Camera fix

This fixes the third person camera being silly around intangible PolyObjects, particularly the fact they can affect the camera's floorz and ceilingz even though I see no reason why they should do so.

Also a good reminder that POF_SOLID is the same as POF_CLIPLINES and POF_CLIPPLANES combined, which is probably how this issue came about to begin with. Can't say that with certainty of course.

See merge request !57

P_ClosestPointOnLine changes

P_ClosestPointOnLine can now (optionally) take custom coordinates for two points that you want to make up a "line", instead of just actual linedefs.

Complete syntax for P_ClosestPointOnLine as of this branch:
```lua
-- syntax for a real linedef (which is already in 2.1.14):
	P_ClosestPointOnLine(x, y, line)

-- syntax for a fake line with vertexes (x1,y1) and (x2,y2):
	P_ClosestPointOnLine(x, y, x1, y1, x2, y2)
```

See merge request !61