Free chasecam improvements and fixes

This adds improvements and fixes to the free chasecam added to 3.2 a while ago:

• The player is no longer looking around themselves while using the chasecam (i.e. the angle and pitch of the free chasecam's camera can change, but not the player's own body).
• Fixed sounds panning incorrectly if looking anywhere except behind the player with the free chasecam (addresses https://zandronum.com/tracker/view.php?id=4044)
• Fixed the free chasecam's view being "locked" while the player is dead (addresses https://zandronum.com/tracker/view.php?id=4061)

Some other important information worth noting:

• To reduce code duplication, the code used to update a player's angle and pitch have also been moved into separate helper functions. This was done in a separate commit.
• To ensure that the local player's view also doesn't change during demo playback, I added a new demo command that toggles the FreeChasecam::enabled boolean, providing a means of checking when the local player was using the free chasecam during the recording.