added missing extern keyword for ms_RoomId in mserv.h (the definition is already in the .c file in this case)

turn all non-extern variables in s_sound.h into externs (and put their real definitions in the .c file)

This fixes act names not being shown on player 2's view

Last minute OpenGL fix: don't check flippedness in HWR_RotateSpritePolyToAim if the mobj is actually a precipmobj!

precipmobj_t does not have eflags, so P_MobjFlip checking it would actually be accessing memory addresses beyond the end of the struct

These fixes were suggested by Sveciaost on #mac-users on Discord