Layer compositing under scaling or rotation bleeds fully obscured layers

[Onne Gorter]

Two layers with identical sizes and transforms and placing, the top layer should fully obscure the first. But instead, depending on scaling or rotations, there is horrible bleeding. Firefox works except under rotation. 3d rendering api's work this way.

example html:
```
<!--
Demonstration of bleeding in layer composition.
1) Remove scale3d() from the outer div, or using "nicer" scales, and there is no bleeding.
2) Setting rotateZ(1deg) on the inner divs and there is also bleeding.
(Notice mozilla only has bleeding for case 2.)
-->
<div id="outer" style="
    position: absolute;
    width: 1000px;
    height: 1000px;
    transform: translate3d(50px, 50px, 0px) scale3d(0.61, 0.61, 1);
    background-color: rgba(255, 220, 220, 1);
">
    <div id="inner1" style="
        position: absolute;
        width: 400px;
        height: 500px;
        transform: translate3d(10px, 10px, 0px) rotateZ(0deg);
        background-color: rgb(0, 0, 0);
        "></div>
    <div id="inner2" style="
        position: absolute;
        width: 400px;
        height: 500px;
        transform: translate3d(10px, 10px, 0px) rotateZ(0deg);
        background-color: rgba(220, 255, 220, 1);
        "></div>
</div>
```

[Simon Fraser (smfr)]

Please attach a test case as an html file, and screenshots.

[Onne Gorter]

Created attachment 313280 [details]
test file showing bleeding

[Onne Gorter]

Created attachment 313281 [details]
screenshot of bleed, the black lines are not supposed to be there

[Onne Gorter]

Created attachment 313283 [details]
example of bleed when rotated

[Onne Gorter]

Created attachment 313284 [details]
screenshot of bleed when rotated, the black lines are not supposed to be there

[Onne Gorter]

Created attachment 313285 [details]
nested layers result in the same bleeding