Fix undefined alpha_scissor in standard shader

[jsjtxietian]

Fixes #95514

[Calinou]

Tested locally, this fixes the shader compilation error.

Testing project: Non-Overlapping Transprancy 2.zip

However, in terms of visuals, it still doesn't look as I'd expect it to. I would expect the shadow opacity to affect the alpha scissor threshold (and ideally not force alpha transparency if alpha scissor transparency is chosen, so you can avoid transparency sorting issues inherent to alpha transparency).

The same goes for alpha hash, although I don't expect this one to be needed as often in this scenario.

Transparency = Alpha (or Disabled, as alpha is forced here)

Transparency = Alpha Scissor

Transparency = Alpha Hash

Transparency = Depth Prepass

With an alpha albedo texture applied:

Transparency = Alpha

Transparency = Alpha Scissor (threshold = 0.2)

Transparency = Alpha Hash

Transparency = Depth Prepass

[clayjohn]

@Calinou Can you open an issue with the extra things you noticed? I think this PR should just be for fixing the compilation error

[Calinou]

@Calinou Can you open an issue with the extra things you noticed? I think this PR should just be for fixing the compilation error

Done: #95602

[akien-mga]

Thanks!

[akien-mga]

Cherry-picked for 4.3.1.