Tweaks to get_unit_normal

manimlib/shaders/inserts/get_unit_normal.glsl

@@ -1,20 +1,20 @@
 vec3 get_unit_normal(in vec3[3] points){
     float tol = 1e-6;
     vec3 v1 = normalize(points[1] - points[0]);
-    vec3 v2 = normalize(points[2] - points[0]);
+    vec3 v2 = normalize(points[2] - points[1]);
     vec3 cp = cross(v1, v2);
     float cp_norm = length(cp);
     if(cp_norm < tol){
         // Three points form a line, so find a normal vector
         // to that line in the plane shared with the z-axis
         vec3 k_hat = vec3(0.0, 0.0, 1.0);
-        vec3 new_cp = cross(cross(v2, k_hat), v2);
+        vec3 comb = v1 + v2;
+        vec3 new_cp = cross(cross(comb, k_hat), comb);
         float new_cp_norm = length(new_cp);
         if(new_cp_norm < tol){
             // We only come here if all three points line up
             // on the z-axis.
             return vec3(0.0, -1.0, 0.0);
-            // return k_hat;
         }
         return new_cp / new_cp_norm;
     }