`Features::SHADER_PRIMITIVE_INDEX` should be supported on all native platforms

[teoxoy]

Naga's glsl, hlsl, msl and spv backends support Builtin::PrimitiveIndex but wgpu only advertises support for Features::SHADER_PRIMITIVE_INDEX on the Vulkan backend.

[cwfitzgerald]

Is there anything we need to do for this, or do we just flip the switch?

[teoxoy]

I think it is a matter of just flipping the switch, yeah.