Fix draw order of transparent materials with multiple directional lights

[mortarroad]

Fixes #47078

[mortarroad]

Never mind, this doesn't seem to work in all cases yet.

[mortarroad]

OK, now it also respects the material priority among multiple transparent shaded materials.
I'm not sure if we really want to support that too; I was mostly concerned about unshaded materials being drawn below shaded ones.
Though it is pretty useful to avoid flickering due to sorting.

[clayjohn]

On a preliminary review, this looks quite good. I need to spend more time reviewing before I approve though.

I have a few performance concerns in the back of my mind, but I think overall it shouldn't be too bad as not many people use multiple directional lights.

[mortarroad]

Another related issue, that does not get fixed by this (i.e. it did not work before, nor does it now):
If a MeshInstance is on a layer with a directional light, but not on the layer with the first directional light in the scene, it will only be drawn in the additive pass.

That means you can't have a scene with two transparent meshes and two lights, with the first mesh being lit by the first light and the second mesh lit by the second light.

Fixing that would require grouping the elements by lights...
We should definitely document that there are issues with transparent objects being lit by directional lights.

[JFonS]

The code makes sense. I did some testing and everything seems to work as expected :)

I don't know if it's worth merging for 3.3 though. On one side, if we are fixing multiple directional lights in 3.3 we might as well do the best we can. On the other, we are introducing more chances for regressions in a late release candidate, for something that is not very widely used...

tl;dr
Looks great, we need to decide if we merge before or after 3.3 :)

[akien-mga]

I think it's better to play it safe and merge for 3.4. The two commits should be squashed into one I guess (or if the second one makes sense as standalone, it should have a clearer title).

[mortarroad]

Squashed and rebased to latest 3.x.

[akien-mga]

Thanks!