Support multiple layers for find_source

[qsniyg]

Thanks to Noodlers on discord for the idea:

there is an issue with small background images overriding links
with Exclude background-images setting disabled (default) i found that if you have an html link pointing to, say, a youtube url and it has a background css set to some data: url in a ::before pseudo element it will try to expand that data: url instead of following to the youtube url and fail to open a popup with

userscript.user.js:111293 Image too small to popup (16x12)

it might be desirable for it to try to open a background image but i think if it fails it should try to continue searching for something else to expand
but also, i feel like an option for disabling popping up data: and blob: urls should exist as well
here is a minimal example:

<style>
.linkify.youtube::before{
    content: "";
    background: transparent url(data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAABAAAAAMCAYAAABr5z2BAAABIklEQVQoz53LvUrDUBjG8bOoOammSf1IoBSvoCB4JeIqOHgBLt6AIMRBBQelWurQ2kERnMRBsBUcIp5FJSBI5oQsJVkkUHh8W0o5nhaFHvjBgef/Mq+Q46RJBMkI/vE+aOus956tnEswIZe1LV0QyJ5sE2GzgZfVMtRNIdiDpccEssdlB1mW4bvTwdvWJtRdErM7U+8S/FJykCRJX5qm+KpVce8UMNLRLbulz4iSjTAMh6Iowsd5BeNadp3nUF0VlxAEwZBotXC0Usa4ll3meZdA1iguwvf9vpvDA2wvmKgYGtSud8suDB4TyGr2PF49D/vra9jRZ1BVdknMzgwuCGSnZEObwu6sBnVTCHZiaC7BhFx2PKdxUidiAH/4lLo9Mv0DELVs9qsOHXwAAAAASUVORK5CYII=) center left no-repeat !important;
    padding-left: 18px;
}
</style>
<a class="linkify youtube" rel="noreferrer noopener" target="_blank" href="https://youtu.be/KkeJkxAMbeI">https://youtu.be/KkeJkxAMbeI</a>