Split StandardMaterialFlags in two for compatibility with low end GPUs

[oscrim]

Objective

Fixes #10066

Solution

Added StandardMaterialAlphaModeFlags containing the alpha mode bitflags previously in StandardMaterialFlags. This removes all the bits previously in the last 16-bits of the StandardMaterialFlags u32 which makes transparency function as intended again.

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[oscrim]

Ive tested this fix on my phone which uses an Adreno 660 which previously couldnt render transparency but with this fix it can.

[mockersf]

could you add comments in the code explaining why it's needed and we can't have everything in a single u32?

[oscrim]

could you add comments in the code explaining why it's needed and we can't have everything in a single u32?

Absolutely, would something like this suffice?

// NOTE: These bitflags are separate from StandardMaterialFlags due to a bug on some older GPUs
// that are unable to use the last 16 bits of a u32
bitflags::bitflags! {
    /// Bitflags info about the alpha mode of the material a shader is currently rendering.
    /// This is accessible in the shader in the [`StandardMaterialUniform`]
    #[repr(transparent)]
    pub struct StandardMaterialAlphaModeFlags: u32 {
        const ALPHA_MODE_OPAQUE          = 0;
        const ALPHA_MODE_MASK            = 1;
        const ALPHA_MODE_BLEND           = 2;
        const ALPHA_MODE_PREMULTIPLIED   = 3;
        const ALPHA_MODE_ADD             = 4;
        const ALPHA_MODE_MULTIPLY        = 5;
    }
}

Also this in pbr_types.wgsl

    // 'flags' is a bit field indicating various options. 
    // u32 is 32 bits so we would have up to 32 options. Some lower-end mobile GPUs only support 16-bits
    // so we can only use half of the u32 
    flags: u32,
    alpha_mode_flags: u32,

[oscrim]

@alice-i-cecile Can I ask why this is considered controversial? And is there anything I can do to where its not controversial?

[alice-i-cecile]

Twiddling with bitflags for hardware-specific support is always a bit scary. It looks like this is a good approach, but it requires more testing and expert opinion than the average rendering PR as a result.

Hmm, actually I think Complex is a better tag here. Going to wait for rendering expertise and testing before merging :)

[valaphee]

Might be worth creating an issue in wgpu (as it could be poly-filled/worked around there)

[oscrim]

Might be worth creating an issue in wgpu (as it could be poly-filled/worked around there)

I created an issue gfx-rs/wgpu#5318

Going to wait for rendering expertise and testing before merging

Is there anything I can do to help out with the testing?