Investigate tolerances of Player->checkGroundState() (more accurate onGround checks)

[dktapps]

Problem description

Currently, players who are as far as 0.2 blocks above the ground will be considered "on ground", as seen here:

PocketMine-MP/src/player/Player.php

Lines 1235 to 1236 in 9cab72e

	$bb->minY = $this->location->y - 0.2;
	$bb->maxY = $this->location->y + 0.2;

This was necessary in the days before PlayerAuthInputPacket as the movement updates were sent at a maximum rate of once per tick, meaning a jumping player could hit the ground and bounce back up again in between packets arriving on the server.

Proposed solution

It may be possible to reduce this tolerance in newer versions, which would fix issues like the one seen in #6500.
It depends whether or not the client consistently sends a movement update upon hitting the ground. This wasn't the case with MovePlayerPacket, but might work better with PlayerAuthInputPacket.

Alternative solutions that don't require API changes

[dktapps]

Looks like this is also a performance issue for servers with large numbers of players.

The current best solution appears to be to use the velocity from PlayerAuthInputPacket to do classic collision checks. (These checks are currently skipped as keepMovement is set to true for players - we just allow the player to move where it wants to.)
However it's a bit awkward since the PlayerAuthInputPacket reports the attempted velocity for the next tick instead of the current one.
In addition, bringing back AABB collision checks for players will probably degrade performance, so it's a trade-off.