Kart cmd->latency port

Nev3r was talking about something that would've been drastically improved with this, and it is really simple, so I ported it :)

ticcmd_t now has a latency field, which gives you how many tics elapsed since it was generated. This is useful for making things adjust in netplay latency, such as timing windows.

added SRB2Kart port label

added Feature label

Unfortunately this is a breaking change - it needs a backwards compat rigging for existing domos, where its non-presence is assumed to be 0 latency. Otherwise immaculate.

I'm not sure @toaster , since previously recorded demos won't make use of this feature at all since it just didn't exist to begin with. Doesn't that mean we shouldn't worry about that?

OH, I didn't see the Z_LATENCY flag, don't mind me :everythingisok:

Yeah, the ZT_LATENCY flag would prevent it from being written or read if it's unchanged, which means it'll just act as 0 in old replays. I just added it just in case anyway, since I'm not sure if it can be non-0 in offline replays, and SRB2 doesn't have online replays. (Although I haven't tested it, so it might also happen if your game lags, but that should be fine anyway.)

changed milestone to %2.2.10

merged

mentioned in commit 786516fa