Fix initial offset calculation when reversed in chase.py

[hberg32]

Found a bug in the chase animation when reverse is set to True which results in the animation being offset by the length of the size parameter. You can observe this by setting up two animations of equal length on a single strip of pixels with the second one reversed. The bars should appear to meet in the middle but don't. We could consider removing _num_repeats and _overflow as they don't appear to be used (at least, searching this repo didn't turn up any hits)

Example:

pixels = neopixel.NeoPixel(board.A0, 22, auto_write=False, brightness = .5) left_pixels = PixelSubset(pixels, 0, 11)
right_pixels = PixelSubset(pixels, 11, 22)
left_animation = Chase(
    left_pixels, speed=.5, color=CYAN, size=2, spacing=11
)

right_animation = Chase(
    right_pixels, speed=.5, color=CYAN, size=2, spacing=11, reverse=True
)

animations = AnimationSequence(
    AnimationGroup(
        left_animation,
        right_animation,
        sync=True
    ),
    auto_clear=True,
    auto_reset=True,
)

while True:
    animations.animate()

[dhalbert]

This makes sense to me: it was a case that was not considered. Thanks!