Investigation: animation / sub-images

[fintelia]

Various image formats have ways of representing multiple images within a single file, either as frames of an animation, different sizes of the same image, and/or multiple distinct images. Currently, we only exposes the AnimationDecoder that lets you turn an image decoder into a sequence of Rgba8 frames, but doesn't have any other functionality for working with files containing multiple sub-images. Given the prevalence of this capability, it may make sense to make it part of the core ImageDecoder trait.

Format	Meaning of sub-images
PNG	animation
JPEG	n/a
GIF	animation
BMP	n/a
ICO	multiple sizes of the same image
TIFF	multiple distinct images or multiple sizes of the same image
WebP	animation
AVIF	animation
PNM	n/a
DDS	multiple images layers with the same format and dimensions
TGA	n/a
OpenEXR	multiple distinct images each potentially with multiple sizes
farbfeld	n/a

^{(Please let me know if you notice any mistakes in the table. I'm not super familiar with most of these formats.)}