Function mat_t 

pub fn mat_t(
    c: usize,
    r: usize,
    tplt_ty: &Type,
    args: &[Instance],
    stage: ShaderStage,
) -> Result<Instance, Error>

matCxR<T>() constructor.

Expects tplt_ty to be valid (a float).

Reference: https://www.w3.org/TR/WGSL/#mat2x2-builtin