added a new function lookAt.

lookAt.glsl

@@ -0,0 +1,10 @@
+#pragma glslify: rotateAxisAngle = require(./rotateAxisAngle);
+
+vec3 lookAt(vec3 base, vec3 p1, vec3 p2, vec3 up) {
+  vec3 diff = normalize(p2 - p1);
+  float d = dot(diff, up);
+  vec3 c = cross(p1, p2);
+  float a = acos(d);
+  return rotateAxisAngle(base, c, a);
+}
+#pragma glslify: export(lookAt)