qbsp: add a placeholder side_t that's just a copy of fae_t

include/qbsp/brush.hh

@@ -28,6 +28,33 @@
 
 class mapentity_t;
 
+struct side_t
+{
+    winding_t w;
+    std::vector<size_t> edges; // only filled in MakeFaceEdges
+    std::optional<size_t> outputnumber; // only valid for original faces after
+                                        // write surfaces
+
+    int planenum;
+    planeside_t planeside; // which side is the front of the face
+    int texinfo;
+    twosided<contentflags_t> contents;
+    twosided<int16_t> lmshift;
+
+    qvec3d origin;
+    vec_t radius;
+
+    // filled by TJunc
+    std::vector<face_fragment_t> fragments;
+
+    // fixme-brushbsp: move to a brush_side_t struct
+    bool onnode; // has this face been used as a BSP node plane yet?
+    bool visible = true; // can any part of this side be seen from non-void parts of the level?
+                         // non-visible means we can discard the brush side
+                         // (avoiding generating a BSP spit, so expanding it outwards)
+    portal_t *portal;
+};
+
 struct bspbrush_t {
     /**
      * The brushes in the mapentity_t::brushes vector are considered originals. Brush fragments created during