MainFrm: Cosmetics for timers.

- Move DVD timing related things to TIMER_STREAMPOSPOLLER for better
  reliability.
- Save current file position if it differs more than 30s than previous.
  In case when mpc-hc is not properly closed.
Fix possible freeze during DVD playback on XP after
f2b6ade8e8185fdc5908ed88df31e68a106786a9

Refs #4299
Do not try to set channel from WebServer thread.

Fixes #4415
CSubPicQueueNoThread: Respect invalidation time.
Do not clear surface cache when max texture size has not been changed.

We were removing valid surfaces from the cache for PGS/DVB subtitles.
Basically surface cache was broken.
SubPic queue: Clear queue only when it's needed.

Time can decrease even within the same video frame and
it make no sense to purge whole queue, if we still can reuse buffered
subpic.