Ideally we need to support perspective-correct rasterization in SW-WR. However, this feature is generally expensive to support in all potential shaders, so we want to have a specific path in SWGL that is only used on some shaders. To that end there would need to be some cleanup in WebRender itself to ensure that only specific shaders trigger the use of perspective and that these are known ahead of time so that SWGL can preprocess and deal with these shaders specially.

This would encompass properly handling the perspective divide for Z values, near plane clipping and potentially also far plane clipping, and most likely using an explicit "smooth" keyword on attributes to signal attributes that require perspective-correct interpolation.

https://hg.mozilla.org/mozilla-central/rev/88a4ebce9de0