ModifyStageVar unintentionally changing p1startx

[RealFoobs]

Describe the bug

ModifyStageVar is changing the starting position of p1 under specific circumstances, interfering with intros.

To Reproduce

1. Add this code to KFMZ (any modifystagevar parameter yields the same results):
   [Statedef -4]
ignorehitpause {modifystagevar{}}
2. From a command line, start a KFMZ mirror match.
3. P1 KFMZ will start his intro right on P2 KFMZ's face, and will warp back to the regular starting position before roundstate = 2.

Expected behavior

The intro should just work as normal.

Screenshots / Video

Engine Version (or source code date)

Current nightly (2024-04-01)

Operating system

Windows

Extra context or search terms

Now here's the messed up part:

This only happens the first time you the load the engine. Hitting F4 or Shift+F4 afterwards does cause the bug to happen again. Adding a check for fighttime > 0 prevents this bug. It's possible ikemen operates on "special" logic on the very frame of a fight.

[Jesuszilla]

Changing oldTickCount to start at 0 rather than -1 in resetFrameTime() seems to fix this?!

EDIT: Fix I applied on top of fantasma's build from #952 seems to behave different from the nightly when applied; it screws with the camera in the latter...

[potsmugen]

I've been wondering if we shouldn't just make ModifyStageVar do nothing in the first frame of the stage, instead of modifying stage code to accomodate it.