Avoid snail trail fading to when white not on white background

[IanMayo]

The current snail trail fades to the background colour of the plot.

This looks unusual when rendered over a blue ocean, like we have with the Natural Earth backdrop.

It would be better if:
a) fade to light blue when Natural Earth is loaded
b) fade to transparent

Option b) would be a better solution, since it would work for a range of backdrop types.

The fix to this issue is probably in SnailDrawTrack.PhasedColorDef, and involves increasing the alpha on the color instead of making the color a steadily closer to the background color.

Note: this workaround for the issue has been passed to analysts: https://www.youtube.com/watch?v=yxx19gwoGfY