This PR adds an option to specify custom colors for a
motion path. One for frames before the current frame
and one frame for after. With this it is easier to see
the relation of the motion path to the current frame.
That was already the case with the default colors, but
not with custom colors.
On a technical side note, the colors pre and post the current
frame were already different.
The shader multiplied the custom color by 0.25
for anything pre current frame.
Pull Request: https://projects.blender.org/blender/blender/pulls/119375
883af6dd63