FGManager: Update VSFilter/XySubFilter blocking routines

No longer disable VSFilter with madVR + XySubFilter, and add XySubFilter's
autoloader to be blocked when ISR is enabled as newer versions depend on
that.
Always prefer internal subtitle renderer

ISR does work fine not only with EVR so it can be safely used on XP also.
Fix ISR loading

- Fix ISR being auto-loaded when it shouldn't be.
- Fix ISR loading when auto-loading is disabled.
Disable some UI elements when ISR is unavailable.
Apply astyle
Prevent ISR from loading if VSFilter is already present.
Rename "Auto-load subtitles" to "Use the built-in subtitle renderer"

"Auto-load subtitles" option has been abused to disable ISR for years.
It makes more sense to change its name. Yet we still load ISR if user try to
load subtitles without any other subtitle renderer present in the graph.
ISR: Fix subtitle timing when external timer and madVR is used.

Partially fixes #4141
Merge pull request #132 from m0vie/avisynth

Use LAV Splitter by default to demux AviSynth script.

This avoids using the old VFW interface that is said to be less stable than directly using AviSynth through ffmpeg. Note that this means AviSynth is still required to demux the scripts.