feat(web): preload assets in photo-viewer

[samholton]

Adding preloadAssets = writable<AssetResponseDto[]>([]); to asset-viewing.store to allow one or more assets to be preloaded after the primary asset has been loaded.

Design choices:

1. Only preload assets if they are of type AssetTypeEnum.Image, didn't want to interfere with video streaming
2. Only preload assets if primary asset is image - didn't want to interfere with video streaming
3. For stacks, preload the next image as well as the second image in the stack
4. Does not preload when initially selecting an asset - although we could preload both the previous and next until we know which direction they are going

The actual loading is currently in photo-viewer because it has the information when the primary is finished loading as well as the helper downloadRequest. Although maybe it makes sense to add an event when primary loading is done and do pre-loading in asset-grid or asset-viewer?

Tested by clearing cache, not enabling disable cache, and setting throttling to a slower connection. Tested both navigating around as well as slideshow.

Does not address the issue in Firefox and the XHR abort not happening.

before without preload
before.webm

after with preload
after with preload.webm

[jvgoch]

Great feature! Unfortunately it doesn't work when the slideshow is set to shuffle. Should I create an issue on the github for this?

[samholton]

Will see if I can take a look this weekend at preloading when in shuffle mode