Metadata access when decoding images

This specification proposes to demux and decode image files. In addition to be able to display the file, it needs to be able to surface frequently used information about those images. A few query in a search engine shows that this is something overwhelmingly popular (for example on stack overflow, but also searching github for wasm/js based decoders).

What immediately comes to mind is to know the dimension of an image before having decoded it (to prepare layout). But generally lots of info are parsed by the implementation, and then not surfaced to script, which is unfortunate, because it means that developers need to ship a bunch of code (probably compiled to WASM or something like that), to do something that the browser already does (and probably better/faster).

Other things I'm thinking of (not exhaustive):

• pixel bit depth / decoded image format (to prepare textures/canvas while/before the image is decoded)
• orientation / more EXIF metadata or other metadata (depending on the format), this is simply necessary to allow layering
• color space (≠ color space conversion)

No file, test, or entry point is named. Start by reviewing the WebCodecs image-decoding specification and define the metadata scope, including dimensions, bit depth, orientation, and color space, before implementation; done means the selected information is surfaced to script.