Background color (or fade-to color) for Chase and Comet

Chase, and Comet, default to a (0, 0, 0) background color. It would be nice to be able to specify a background color or fade-to color, besides the default (0, 0, 0) color. This would let the user differentiate their display giving it a less generic look. And with Grouped (layered) animations, when the top animation overides the bottom animation, without using (0, 0, 0) colored pixels, it might be more pleasing look.

Start by locating the Chase and Comet animation entry points in the Adafruit_CircuitPython_LED_Animation repository and reviewing how their default (0, 0, 0) background is applied. Define the requested background or fade-to color behavior for standalone and grouped animations, then verify that both animation types and layered overrides use the selected color.

Written by the indexing model from the issue text.