amp-img won't render when nested in a hidden element

If you have a bug for AMP please fill in the following template. Delete everything except the headers (including this text).

## What's the issue?
amp-img won't render

## How do we reproduce the issue?

Try the following example on the playground https://playground.amp.dev/amp/

```

My AMP Page


body {
-webkit-animation: -amp-start 8s steps(1, end) 0s 1 normal both;
-moz-animation: -amp-start 8s steps(1, end) 0s 1 normal both;
-ms-animation: -amp-start 8s steps(1, end) 0s 1 normal both;
animation: -amp-start 8s steps(1, end) 0s 1 normal both
}

@-webkit-keyframes -amp-start {
from {
visibility: hidden
}
to {
visibility: visible
}
}

@-moz-keyframes -amp-start {
from {
visibility: hidden
}
to {
visibility: visible
}
}

@-ms-keyframes -amp-start {
from {
visibility: hidden
}
to {
visibility: visible
}
}

@-o-keyframes -amp-start {
from {
visibility: hidden
}
to {
visibility: visible
}
}

@keyframes -amp-start {
from {
visibility: hidden
}
to {
visibility: visible
}
}

body {
-webkit-animation: none;
-moz-animation: none;
-ms-animation: none;
animation: none
}











.hidden {
display: none
}

#myinput:checked + .hidden {
display: block
}

MyInput

```

Please provide a public URL and ideally a reduced test case (e.g. on jsbin.com) that exhibits only your issue and nothing else. Provide step-by-step instructions for reproducing the issue:

1. Here we have a deeply nested amp-img in an initially hidden div.
2. When clicking on the checkbox the div will become visible, but the amp-img won't load or it will load randomly.


## What browsers are affected?

I've tried on firefox / chrome and get consistent behaviour in the playground

## Which AMP version is affected?

Powered by AMP ⚡ HTML – Version 2104170104001

Reproduce the nested amp-img case in the AMP Playground using the provided HTML and checkbox steps, then trace the amp-img behavior when its parent changes from display:none to visible. Done means the image renders consistently after the hidden container is revealed in Firefox and Chrome.

Written by the indexing model from the issue text.