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/slic3r/GUI/Jobs/TextureDisplacementPreviewJob.cpp

@@ -24,6 +24,8 @@ void TextureDisplacementPreviewJob::process(Ctl &ctl)
     TextureColorRequest *color = nullptr;
     if (!m_input.color.empty()) {
         color_request.quantize = GLGizmoTextureDisplacement::make_palette_quantizer(m_input.color.palette);
+        if (!m_input.color.palette_pure.empty())
+            color_request.quantize_pure = GLGizmoTextureDisplacement::make_palette_quantizer(m_input.color.palette_pure);
         color_request.resolve  = GLGizmoTextureDisplacement::make_mix_resolver(
             m_input.color.palette, m_input.color.mix_mode, m_input.color.layer_height,
             m_input.color.dither_cell_mm);