Icon changes/fixes

This addresses a couple of issues with rendering icons above the player's head:

1. Fixed icons not rendering above the head of players being spied on while the local player's in chasecam mode, simply because the player being spied on isn't using the chasecam themselves (https://zandronum.com/tracker/view.php?id=401).
2. Ally/enemy icons no longer render above the player being spied on (e.g. you). I don't think it's necessary to display these icons over our own heads, and it should only show up for other players according to our perspective (was supposed to address https://zandronum.com/tracker/view.php?id=4296 before it closed).
3. Fixed icons "flickering" over the player's head when they're just about to respawn in offline games (https://zandronum.com/tracker/view.php?id=4281). The behaviour of icons in offline and online games is more consistent now.
4. Even when the local player's not in chasecam mode, icons can now appear over the head of the player being spied on when it's being rendered in a mirror (https://zandronum.com/tracker/view.php?id=4302).

NOTE: it's better to replace this change with the ONLYVISIBLEINMIRRORS actor flag from GZDoom (https://github.com/ZDoom/gzdoom/commit/593627f0499ce3cda972368fe07dacaa00eac68d) when we backport it in the future. I left a comment as a reminder.

5. The duration of medals no longer tick while the game's paused, so medals don't abruptly disappear in this state.