Cleanup: Simplify vector direction comparison

Multiply the two number together. If the sign of the result is positive, then the sign was the same. If the sign of the result is negative, then the signs were different.

source/blender/editors/gpencil/gpencil_interpolate.c

@@ -237,8 +237,8 @@ static bool gpencil_stroke_need_flip(Depsgraph *depsgraph,
     float v1[2], v2[2];
     sub_v2_v2v2(v1, v_from_end, v_from_start);
     sub_v2_v2v2(v2, v_to_end, v_to_start);
-    if (((v1[0] > 0.0f && v2[0] < 0.0f) || (v1[0] < 0.0f && v2[0] > 0.0f)) &&
-        ((v1[1] > 0.0f && v2[1] < 0.0f) || (v1[1] < 0.0f && v2[1] > 0.0f))) {
+    mul_v2_v2v2(v1, v1, v2);
+    if ((v1[0] < 0.0f) && (v1[1] < 0.0f)) {
       return true;
     }
   }