Refactor alpha_discard for reuse with different material types

[bonsairobo]

Objective

I want to call the pbr_functions::alpha_discard shader function from my own material shader, but it only takes a StandardMaterial parameter.

Solution

This PR replaces the StandardMaterial parameter with less restrictive ones so my shader can call it.

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Changelog

• The signature of pbr_functions::alpha_discard changed by replacing StandardMaterial with only the flags and alpha_cutoff fields.

Migration Guide

Replace this:

pbr_functions::alpha_discard(material, ...);

with this:

pbr_functions::alpha_discard(material.flags, material.alpha_cutoff, ...);

[superdump]

Could you rework prepass_alpha_discard() in the same kind of way?

[bonsairobo]

@superdump

Could you rework prepass_alpha_discard() in the same kind of way?

Sure, if you don't mind that I extract a new function for only lines 47-60. The first part of that function does some sampling using global binding variables, and that defeats the point of making it reusable IMO.

[bonsairobo]

Merged with bevy 0.12, ready for review again.