Slideshow advance displays images in the wrong order using PlaybackDirection

[xnkevinnguyen]

When setting the playbackdirection to forward then backward, the slideshow will not have a consistent order.

Repro code:

 from adafruit_clue import clue
from adafruit_slideshow import SlideShow, PlayBackDirection

slideshow = SlideShow(clue.display, folder="img", auto_advance=False)

while True:
    if clue.button_b:
        slideshow.direction = PlayBackDirection.FORWARD
        slideshow.advance()
    if clue.button_a:
        slideshow.direction = PlayBackDirection.BACKWARD
        slideshow.advance()

Actions: click button b, then a,
current: image 1 => image 2 -> image 3
expected: image 1 -> image 2 -> image 1

[andreamah]

Fixed in #350