cofactor matrix, as described by https://github.com/graphitemaster/normals_revisited

cofactor.glsl

// cofactor matrix. drop-in replacement for the traditional transpose(inverse(m)) normal matrix calculation.
// this is a substantial performance improvement.
// short form found via shadertoy: https://www.shadertoy.com/view/3s33z
mat3 cofactor(mat4 m) {
	return mat3(
		cross(m[1].xyz, m[2].xyz),
		cross(m[2].xyz, m[0].xyz),
		cross(m[0].xyz, m[1].xyz)
	);
}