Fragment shader integer built-ins incorrectly require `flat`

[Firestar99]

Integer Built-ins in fragment shaders currently require the flat decoration, like in our compiletest:

rust-gpu/tests/compiletests/ui/spirv-attr/all-builtins.rs

Lines 96 to 100 in a30bd43

	#[spirv(primitive_id, flat)] primitive_id: u32,
	#[spirv(sample_id, flat)] sample_id: u32,
	#[spirv(sample_mask, flat)] sample_mask: [u32; 1],
	#[spirv(sample_position)] sample_position: Vec2,
	#[spirv(viewport_index, flat)] viewport_index: u32,

otherwise compile will fail with:

error: `Fragment` entry-point `Input` parameter must be decorated with `#[spirv(flat)]`
    --> crates/restir-shader/src/visibility/raster.rs:40:39
     |
  40 |     #[spirv(primitive_id)] primitive_id: u32,
     |                                          ^^^

However, decorating them with flat makes no sense and may even be illegal, since they are builtins, and flat configures interpolation behaviour.

[Firestar99]

I'm willing to leave this one as a "won't fix" since we're discussing a whole entry point redesign anyway