Blender Git Loki

Image scopes: Make sample line saving check local

Previously it was using accumulative counter of saved lines and so on in order
to detect cases when new sample is to be saved. This is not quite possible to
do with threaded scopes update.

Change it now with non-accumulative approach which saves a bit different lines
due to slightly different rounding, but this things are not strictly defined
anyway and results are close enough to each other.