[3.2] Workaround for alpha scissoring with opaque pre-pass in SpatialMaterial

[Avogine]

This implements a workaround for broken alpha scissoring with opaque pre-pass in 3.2.

Video:
https://user-images.githubusercontent.com/55308941/104935573-af48de00-59ab-11eb-8308-e909c4e7deea.mp4

with workaround:
https://user-images.githubusercontent.com/55308941/104942616-fab3ba00-59b4-11eb-937e-325d82ce2d6d.mp4

This doesn't fix the real issue, because alpha scissoring will still be broken if used with ALPHA_SCISSOR=alpha_scissor_threshold;, but this will be reimplemented in 4.0, so I think this workaround is good enough for now.

(workaround by AsherGlick and Arnklit here: #36669)

MRP: alpha_scissor_workaround.zip

[clayjohn]

I'm not sure about this. This changes behaviour for both GLES2 and GLES3, while it seems like the original bug was GLES3 only.

I also don't like the idea of changing engine source to add a workaround that was suggested for users to add to their own shaders. We would potentially break user's materials who are not running into this bug.

What we need is a PR that fixes the underlying bug which is likely in drivers/gles3/shaders/scene.glsl.

[Avogine]

You are right, I don't like this workaround either, but I don't think I can fix the glsl code, because I'm not really experienced with the source of godot.
I did this in the first place, because all my 3d models with alpha scissor are broken, and as far as I know I can't convert an imported SpatialMaterial into a shader.

[Avogine]

I got another Idea, what if we expose the function to convert a SpatialMaterial to a Shader, so users can do this workaround inside an import script. This is what I wanted to do in the first place, but I wasn't able to find a function for converting.

[Avogine]

I'm closing this, someone wih better experience should fix this properly.