VDecimate: Rearrange the code a bit

I think it's more readable, and it will make it easier to modify frame
properties, now that frames are returned from a single place.

The output and the speed are the same.
VDecimate: Load overrides from createVDecimate

All other initialisation is there.
VDecimate: Add metrics output
VDecimate: Adjust frame durations when they exist
VDecimate: Fix decimation of the first cycle

The first frame's metrics are always zero, thus the first frame is
always considered a duplicate, which means it's always dropped. Avoid
this by copying the second frame's maximum block difference.

Also make the first frame's total difference just a bit higher than the
scene change threshold, so it's always considered a scene change. This
way the first frame will be dropped when there are no duplicates and no
other scene changes in the cycle.
VDecimate: Only copy dst when necessary
Merge pull request #78 from dubhater/stuff

VDecimate additions