AAS_PlayerMovementPrediction() doesn't handle friction correctly

[zturtleman]

AAS_PlayerMovementPrediction() doesn't handle friction correctly

I think I have found a fix for this issue:
It seems that the main problem is how the friction is computed per frame (either onground or swiming). What I tried now is to use the computation from 'PM_Friction' from bg_pmove code instead of the approach from 'AAS_ApplyFriction' inside the bot code.
Strange enough the results are good, the output AAS file now has both, jump reachabilities as well as swim reachabilities.

To make it clearer I will sum up the issue again:
The default bug was that either jump reachabilities was generated (when compiling an AAS file), or correct swim reachabilities was calculated. The movement prediction code is not only used for generating AAS files, it is also used during bot game, which makes the problem even worse.
This bug was already fixed here: ioquake/ioq3@9c4c363 where the specific friction was reversed. This mainly fixed the 'real-time' computation, but NOT the generation of correct AAS files.

I have to admit that I don't really understand why using a modified version 'AAS_ApplyFriction' is fixing the issue, hence I didn't make a PR yet. In other words, if someone is interested in fixing this issue, than I can make a PR, but I would need help than, to better understand whats going on here.
I think it is worth noting that ALL idtech3 engines are concerned!
Anyways here is a snippet how the reworked 'AAS_ApplyFriction' code would look like:

static void AAS_ApplyFriction(vec3_t vel, float friction, float stopspeed, float frametime)
{
float speed, control, drop, newspeed;
//horizontal speed
drop = 0;
speed = sqrt(vel[0] * vel[0] + vel[1] * vel[1]);
if (speed) {
control = speed < stopspeed ? stopspeed : speed;
drop += control * friction * frametime;
newspeed = speed - frametime * drop; // Tobias NOTE: why is 'frametime * ' needed? See 'PM_Friction'.
if (newspeed < 0) newspeed = 0;
newspeed /= speed;
VectorScale(vel, newspeed, vel);
}
}

The important thing seems to be 'newspeed = speed - frametime * drop;'

Additionally I will add a small patch I made for World of Padman as a blueprint:
bspc-botlib-friction-fix-showcase.txt (rename the file from .txt to .patch or .diff,).
Please don't ever apply this file directly as a patch, it was only made for demonstration purposes, to track the issue, though it will also fix the bug, I think. The 'new' solution of reworking the 'AAS_ApplyFriction' function seems more reliable.