Fix playhead snapping to seconds, when the frame rate is less than 0.5
FPS.
This makes it possible to snap to fractions of frames, to support setups
with multiple seconds per frame. Of course this only has any effect
(apart from not crashing) when sub-frames are enabled.
I've also added unit tests, and verified that the values are the same
from before this refactor.
Pull Request: https://projects.blender.org/blender/blender/pulls/148074
bbf40d214c