This happens to fix most of the undo issues, as it's now much harder to
have uncommitted changes to the file.
Closes#355 and #586.
Originally committed to SVN as r4699.
Kill vfr.h and vfr.cpp and use the libaegisub versions of them instead.
Rather than the globals VFR_Input and VFR_Output, everything related to
frame rate is now part of the video context. Most things which used to
use VFR_Output now call VideoContext::TimeAtFrame etc.; video providers,
rather than modifying VFR_Input directly, now have getters for their
frame rates which VideoContext calls. Read-only public access to
VFR_Input and VFR_Output are still provided (hopefully temporarily) for
a few things which were awkward to do through VideoContext.
The Avisynth provider now might correctly handle VFR MKVs which can be
opened with DirectShowSource but not DSS2.
Rework keyframe handling as well, so that it continues to match the vfr
handling in design and implementation.
Originally committed to SVN as r4662.
