Texture displacement: per-layer bake, UV pane redesign, unwrap and layer view fixes

- Bake: each layer is sampled only on its own painted area; analytic
  projections used to stack every layer over every painted region, so the
  top layer's texture showed on all of them (colour sampler too)
- Auto resolution follows the texture's texel size and sharpness again
- Unwrap: charts cut by each face's own normal (a cube gives 6 islands, not
  12 triangles); non-disk charts (tubes, closed shells) are split until they
  flatten; connected nets test real triangle overlap, grow from the largest
  chart and are packed side by side
- UV edits are stored per unwrapped copy, so dragging a seam vertex no
  longer moves its copies in neighbouring islands
- UV pane: tool strip with unwrap settings moved in from the panel, sharp
  HiDPI icons, clearer island/edge/selection drawing with hover, texture
  picker from the thumbnail, texture no longer lost on reopen (GL state
  from the 3D view, background upload retries)
- Panel: whole-model select/erase as icons in the tools row; inactive
  layers' paint shown muted; colour textures shown in colour in the picker
- Built-in displacement texture library
- Tests for unwrap segmentation, connected nets, UV edits and per-layer
  sampling

src/libslic3r/Model.cpp

@@ -2943,6 +2943,11 @@ void ModelVolume::assign_new_unique_ids_recursive()
     seam_facets.set_new_unique_id();
     mmu_segmentation_facets.set_new_unique_id();
     fuzzy_skin_facets.set_new_unique_id();
+    // As set_new_unique_id() already does: the undo/redo stack stores FacetsAnnotation contents keyed
+    // by ObjectID, so a clone left sharing these ids with its source can be handed the source's mask
+    // on an undo - after which a paint mask and the mesh it was recorded against no longer match.
+    for (int i = 0; i < int(TEXTURE_DISPLACEMENT_MAX_LAYERS); ++i)
+        texture_displacement_facet(i).set_new_unique_id();
 }
 
 void ModelVolume::rotate(double angle, Axis axis)