Respawn Certain Monitors in Co-op

Currently, normal grey monitors never ever respawn in Co-op. This makes it so that if one player grabs a shield, all other players in that server are now completely locked out of grabbing that shield. This isn't great when power-up monitors are generally uncommon and used as a reward for finding hidden areas. This merge request addresses that by letting certain monitors respawn. The monitors in question are all shield monitors, speed shoes monitors, invincibility monitors, and gravity shoes monitors.

When a monitor spawns, MF2_DONTRESPAWN is set depending on whether or not the monitor is supposed to respawn. This can be modified in real time via Lua using any hook, including MobjSpawn, as well as via A_SetObjectFlags2 in SOC. This makes it easy to set whether you want your custom monitors to respawn, or simply want to change the vanilla monitor respawning behavior.

In addition, Strong Random Monitors and Weak Random Monitors no longer apply in Co-op. Not that they would normally anyways, seeing as monitors didn't respawn before.

Test exe: monitorrespawn3.exe

Lua to force all monitors to respawn: L_RespawnMonitorsTest.lua