Build a 4x4 look at matrix in cginc

LookAtMatrix.cginc

#ifndef _LOOK_AT_MATRIX_
#define _LOOK_AT_MATRIX_

float4x4 look_at_matrix(float3 at, float3 eye, float3 up) {
	float3 zaxis = normalize(at - eye);
	float3 xaxis = normalize(cross(up, zaxis));
	float3 yaxis = cross(zaxis, xaxis);
	return float4x4(
		xaxis.x, yaxis.x, zaxis.x, 0,
		xaxis.y, yaxis.y, zaxis.y, 0,
		xaxis.z, yaxis.z, zaxis.z, 0,
		0, 0, 0, 1
	);
}

#endif