Fix sampling step alignement

[julienduroure]

Root issue is having a sampling step different than 1, and full object baking enable.

Step is 3
Action starts at 1, so the action will try to export frames 1 / 4 / 7 / 10, etc...
But full object backing is enable, and scene starts at 0.
So cached data are on frame 0 / 3 / 6 / 9 / 12, etc...

When we are trying to export the action on armature after the full bake on empty, we are trying to retrieve frame 1, but cache was on 0 / 3 / 6 / 9

Workaround before I fix it:

Set step at 1 (probably not what you want)
Keep step at 3, but start scene at 1 instead of 0 => This will align the cache to same values