api: remove redundant ImageSpec from ImageCache (issue #4436)

[bfraboni]

Description

First draft of frontend changes for issue #4436.

Checklist:

• change imagespec, get_imagespec API calls in ImageCache, ImageCacheImpl, TextureSystem and all calls.
• add get_cache_dimensions to access cache internal mip level dimensions
• add get_cache_dimensions unit tests once we validate the new version
• change backend to reduce memory usage (might be a different PR)

[lgritz]

Other than the comment I made about elaborating the description a bit, this is all looking great to me.

We talked in a different channel about adding back some very minimal inline methods to keep back compatibility for now, so I know that's still on its way, but if that ends up straightforward, we should be able to merge this without any trouble. Then the API will be nailed down and you can do the further changes to the internals without any additional compatibility breaks during our upcoming beta period.

[bfraboni]

I've added the backwards compatible methods for ImageCache get_imagespec() and imagespec(), and TextureSystem get_imagespec(). I think the CI is happy now, I just need to write unit tests for the newer get_cache_dimensions() method and it'll be good to go.
Thank you @lgritz for the directions !

[bfraboni]

I've just rebased the PR and added the unit tests for get_cache_dimensions()

[lgritz]

LGTM!

I suggested two changes to docs -- just inserting a blank line to be sure it understands the parameter command. I will add those for you by simply accepting the suggestions, then merge.

Thanks, this is great. Looking forward to the eventual change to the internal implementation, but this at least gets the new API locked into place.