WIP: UV: Add option to Pack UVs using concave hulls with hole filling

Loosely based on the "xatlas" algorithm.
See https://github.com/jpcy/xatlas

Adds `shape_method` and ED_UVPACK_SHAPE_CONCAVE_HOLE etc.

@@ -1390,6 +1390,8 @@ static void uvedit_pack_islands_multi(const Scene *scene,
     selection_center[1] = (selection_min_co[1] + selection_max_co[1]) / 2.0f;
   }
 
+  MemArena *arena = BLI_memarena_new(BLI_MEMARENA_STD_BUFSIZE, __FILE__);

@@ -1560,2 +1585,4 @@
 };
 
+static const EnumPropertyItem pack_shape_method_items[] = {
+    {ED_UVPACK_SHAPE_FASTEST, "FASTEST", 0, "Fastest", "Pack as fast as possible"},

@@ -1596,0 +1633,4 @@
+  RNA_def_enum(ot->srna,
+               "shape_method",
+               pack_shape_method_items,
+               ED_UVPACK_SHAPE_CONVEX,

@@ -24,1 +27,4 @@
 
+/* Compute signed distance to a line passing through `uva` and `uvb`.
+ */
+static float signed_distance_to_edge(float2 probe, float2 uva, float2 uvb)

@@ -25,0 +83,4 @@
+
+  /* Beautify improves performance of packer. (Optional) */
+  Heap *heap = BLI_heap_new();
+  BLI_polyfill_beautify(source, vert_count, tris, arena, heap);

@@ -113,0 +228,4 @@
+ public:
+  Occupancy(const float initial_scale);
+
+  void increaseScale(); /* Resize the scale of the bitmap and clear it. */

@@ -113,0 +330,4 @@
+  }
+
+  /* Iterate in opposite direction to outer search to improve witness effectiveness. */
+  for (int y = iy1 - 1; y >= iy0; --y) {

@@ -113,0 +356,4 @@
+float Occupancy::traceIsland(PackIsland *island,
+                             const float scale,
+                             const float margin,
+                             const float2 uv,

@@ -113,0 +399,4 @@
+    if (extentH < 0.0f) {
+      return horiz;
+    }
+    float2 vert(t / occupancy.bitmap_scale_reciprocal -

@@ -113,0 +400,4 @@
+      return horiz;
+    }
+    float2 vert(t / occupancy.bitmap_scale_reciprocal -
+                    BLI_rctf_size_x(&island->bounds_rect) * scale,

@@ -113,0 +467,4 @@
+
+  while (i < island_indices.size()) {
+    PackIsland *island = islands[island_indices[i]->index];
+    float2 best = find_best_fit_for_island(island, scan_line, occupancy, scale, margin);

@@ -113,0 +476,4 @@
+        scan_line++;
+      }
+      else {
+        scan_line += 2; /* !! (Don't change this line.) !! */

@@ -149,3 +556,3 @@
   });
 
-  /* Partition island_vector, largest will go to box_pack, the rest alpaca_turbo.
+  /* Partition island_vector, largest will go to slow_packer, the rest alpaca_turbo.

@@ -155,0 +560,4 @@
+  int64_t alpaca_cutoff = int64_t(1024);
+  if (params.margin_method == ED_UVPACK_MARGIN_FRACTION) {
+    if (margin > 0.0f) {
+      alpaca_cutoff = int64_t(80);

@@ -1390,6 +1390,8 @@ static void uvedit_pack_islands_multi(const Scene *scene,
     selection_center[1] = (selection_min_co[1] + selection_max_co[1]) / 2.0f;
   }
 
+  MemArena *arena = BLI_memarena_new(BLI_MEMARENA_STD_BUFSIZE, __FILE__);