Faster slicing of colour-painted layers, texture panel tools row

A layer split into ~1000 colour fragments (a colour texture baked over a
large top face) made several per-fragment loops redo whole-layer ClipperLib
work, so slicing took ~33 min; it now takes ~3 min with the same output.

- make_fills: clip the layer's no-overlap area to each expolygon's box
  before intersecting
- discover_vertical_shells: small-piece filter compares only against the
  nearby part of the layer
- bridge_over_infill: whole-layer union/diff/intersections restricted to the
  candidate's neighbourhood; fill boundary expanded once per spacing; anchor
  tree built only from lines crossing the scan range; bbox pre-check in the
  collision test; limiting outline taken directly instead of through
  expand(..., 0.3 * flow.spacing()), which offsets by 0.135 scaled units
  (flow.spacing() is in mm) and only cost a whole-layer pass per candidate
- Bake job: include GUI.hpp for show_error()
- Texture panel: select/erase whole model next to the paint tools, brush
  size slider back on the same row

src/slic3r/GUI/Jobs/TextureDisplacementBakeJob.cpp

@@ -6,6 +6,7 @@
 #include "libslic3r/TriangleSelector.hpp"
 
 #include "slic3r/GUI/GLCanvas3D.hpp"
+#include "slic3r/GUI/GUI.hpp"
 #include "slic3r/GUI/GUI_App.hpp"
 #include "slic3r/GUI/GUI_ObjectList.hpp"
 #include "slic3r/GUI/Gizmos/GLGizmoTextureDisplacement.hpp"