qbsp: don't use brush sides to texture a portal if they're not coplanar

include/common/polylib.hh

@@ -988,6 +988,18 @@ public:
         return counts;
     }
 
+    vec_t max_dist_off_plane(const qplane3d& plane)
+    {
+        vec_t max_dist = 0.0;
+        for (size_t i = 0; i < size(); i++) {
+            vec_t dist = abs(plane.distance_to(at(i)));
+            if (dist > max_dist) {
+                max_dist = dist;
+            }
+        }
+        return max_dist;
+    }
+
     /*
     ==================
     ClipWinding

qbsp/portals.cc

@@ -835,6 +835,11 @@ static void FindPortalSide(portal_t *p, visible_faces_stats_t &stats)
             for (auto &side : brush->sides) {
                 if (side.bevel)
                     continue;
+
+                if (p->winding.max_dist_off_plane(side.get_plane()) > 0.1) {
+                    continue;
+                }
+
                 if ((side.planenum & ~1) == p->onnode->planenum) {
                     // exact match (undirectional)