Consider all frames when selecting mode for PNG save_all #6610
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Resolves #6608
At the moment, when saving multiple frames with PngImagePlugin, the mode of the first image is used.
Pillow/src/PIL/PngImagePlugin.py
Line 1224 in af8260e
However, this is not necessarily ideal. For a GIF, the first frame might be P, and subsequent frames might be RGB. Converting the RGB frames to P could result in loss of information.
This PR loops through the frames to be saved, checks if any are RGB, RGBA or P, and upgrades the mode accordingly.