Blender Git Loki

Fix T70381: Motion Paths off by one

The apparent off-by-one error was caused by a few factors:

- The 'blend base' colour was green for the two frames directly
surrounding the current frame, but black for the current frame itself.
- For the frames before the current one, the 'blend base' was mixed with
black, making the green stand out clearly, but fading to black again
for the current frame. This looks like an off-by-one, even though it
was just bad mixing.
- For the frames after the current one, the 'blend base' was mixed with
cyan, which already has a strong green component, so mixing it there
was much less visible, making the entire thing look like an off-by-one
error where it actually wasn't.

I have simplified the code, so now it only chooses green as the 'blend
base' for the current frame, and simplified the mixing for the current
frame.