Heretic multiplayer demo playback (Fix for #1022)

[mykola-ambar]

This PR adresses #1022, adding a command line switch that allows for demo playback in network mode.

[chungy]

Thanks for the contribution but I have a few concerns:

1. Vanilla compatibility

The behavior of the vanilla engine with multiplayer demos might be taken into consideration whether this patch makes sense. It's not a blocker (for example, we already have a working -timedemo where vanilla Heretic and Hexen are incapable), but if vanilla supports multiplayer demos, we ought to make sure we are fully compatible with it. If vanilla doesn't support it but the patch is this trivial, I think we can make a reasonable exception.

2. Extra command line parameter

In my opinion, these demos should "just work" without needing an extra -netdemo parameter.

3. Hexen support

Hexen probably requires a similar patch to this, would be nice to have it.

[mykola-ambar]

IIRC The vanilla executable does not support multiplayer demos. I added a separate command line parameter to make sure the default behavior stays true to the expected vanilla behavior - but I can change it to be the default behavior if you want.
Hexen seems to have the same problem, I'll fix it there as well.

[mykola-ambar]

I tested the vanilla v1.3 executable and it indeed does not support multiplayer demos (i wasn't 100% sure before). I've made the demos just work by default without needing the command line parameter.

I've also added some code to make the status bar correctly show the spectated player's stats, but I'm not sure if this change is "vanilla" enough for chocolate doom, what do you think?

[fragglet]

It's a good question to be asking. I think there's genuine utility in being able to play back multiplayer demos, so it's worth adding. But it's also the case that vanilla Heretic can't play back multiplayer demos. We already have a framework for dealing with this kind of situation in the -strictdemos command line argument. So the correct way to do this is to gate the fix using the D_NonVanillaPlayback() function.

One other thing - please reduce the scope of this change to just fixing playback - it's not necessary to fix the status bar as well.

[Xindage]

i id like to have the executable of this PR with the statusbar bar update if possible, so my friend can properly record our cooperative speedrun so people can see my hp and ammo too.

[jmtd]

The status bar fix should be raised as a separate issue so it can be discussed there.