Improvements to image animation

[AnonymouX47]

• Restores frame caching based on framecount formerly reverted by ea6f74b and 97464fd
• Replaces frame caching based on filesize
• Removes optimizations for animated images which are no longer necessary

Pillow v9.1.0 has been released with the improvements in https://github.com/python-pillow/Pillow/pull/6077