Be more lenient on the vector type used for global_invocation_id

[oisyn]

From @hrydgard: The builtin #[spirv(global_invocation_id)] id: UVec3 needs to be declared as an UVec3, and then truncated if you want a Vec2. In HLSL and GLSL it's common to declare it as a 2-vector instead (UVec2) if you only care about x and y.