python-pillow
Do not update palette for L mode GIF frame
Resolves #8921
When a GIF frame doesn't have a global palette, and an individual frame has a palette where the colours sequentially move from (0, 0, 0) to (255, 255, 255), or even if the palette is less than 256 colours in length and still follows that progression, Pillow reads that as an L mode frame.
https://github.com/python-pillow/Pillow/blob/58e48745cc7b6c6f7dd26a50fe68d1a82ea51562/src/PIL/GifImagePlugin.py#L93-L97 https://github.com/python-pillow/Pillow/blob/58e48745cc7b6c6f7dd26a50fe68d1a82ea51562/src/PIL/GifImagePlugin.py#L291-L294 https://github.com/python-pillow/Pillow/blob/58e48745cc7b6c6f7dd26a50fe68d1a82ea51562/src/PIL/GifImagePlugin.py#L315 https://github.com/python-pillow/Pillow/blob/58e48745cc7b6c6f7dd26a50fe68d1a82ea51562/src/PIL/GifImagePlugin.py#L430
GIF frames may have transparency though, and https://github.com/python-pillow/Pillow/blob/58e48745cc7b6c6f7dd26a50fe68d1a82ea51562/src/PIL/GifImagePlugin.py#L479-L480 is not able to accomodate an L mode frame with transparency.
This fixes that.
