glam v0.33.6 breaks `Vec3A`

[Firestar99]

PR bitshifter/glam-rs#786 changed the definition of Vec3A to make it bytemuck::Pod:

  #[repr(align(16))]
  #[repr(C)]
  #[cfg_attr(target_arch = "spirv", rust_gpu::vector::v1)]
  pub struct Vec3A {
      pub x: f32,
      pub y: f32,
      pub z: f32,
+     _w: f32,
  }

So from rust-gpu's perspective, this is now a vector with 4 floats instead of 3 floats, breaking a lot of safety assumptions in our codebase.

Observed Fallout

• breaks rust-gpu's Matrix4x3 layout, causing certain raytracing-related intrinsics to miscompile into invalid spirv. See compiletest ray_query_get_intersection_object_to_world_khr breaking after glam update, as seen in this CI of PR update to rustc 1.99.0 #630
• This CI of PR adjust Matrix4x3 to unbreak glam 0.33.6 #646 shows compiletest sample_gradient failing, showing potential higher fallout:

error: error:0:0 - Expected Image Operand Grad dx to have 3 components, but given 4
         %38 = OpImageSampleDrefExplicitLod %float %37 %32 %float_1 Grad %33 %34

Mitigation

Problems:

• glam v0.33.6 is a breaking change for rust-gpu
• glam added _w: f32 member to get impl bytemuck::Pod for Vec3A, so reverting this is a breaking change for regular glam users
• don't want impl Pod not to be impl on spirv

Ideas:

• hack something in the compiler to special case glam::Vec3A
  • how do we know if a type is Vec3A? This will be a hack and not really sustainable
• PR adjust Matrix4x3 to unbreak glam 0.33.6 #646 adjusts Matrix4x3 to use Vec3 instead of Vec3A. The layout of that special type is screwed up anyway, so we don't really care about it changing.
  • not a full mitigation!

Strategy:

1. short term fix

• special case spirv to not have _w: f32 member and not impl Pod,
• release glam v0.33.9, fixing rust-gpu

2. long term fix

• glam::Vec3A:
  • remove short term fix
  • change attribute to #[spirv::vector::v1(3, f32)] to overwrite rust-gpu vector type
  • release glam v0.34.0
• rust-gpu:
  • adjust #[spirv::vector::v1] attribute to take params to overwrite vector type
  • make rust-gpu support glam v0.34.0
  • release rust-gpu?

will update this description as I go