Duke3D: camera vec3_t position smoothing causes visual glitches with transport sectors

Commit a359877b added camera vec3_t position smoothing across frames to mask jitter that occurs when strafing and turning at the same time.

However, it causes visual glitches with elevator transport sectors. (and perhaps other instant transport effects as well)

The following map places a sky sector between source and destination to make the glitch more obvious. It however occurs without this as well.

Before the commit: https://cdn.discordapp.com/attachments/461290354726010891/738717075781976144/2020-07-31_12-59-48.mp4

After the commit: https://cdn.discordapp.com/attachments/461290354726010891/738717185387790406/2020-07-31_13-06-02.mp4

Map used: transport_glitch.map