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

resources/images/texture_displacement_erase_all.svg

@@ -0,0 +1 @@
+<svg xmlns="http://www.w3.org/2000/svg" width="16" height="16" viewBox="0 0 16 16"><polygon points="0.5 3 6 5.5 11.5 3 6 0.5 0.5 3" style="fill:none;stroke:#949494;stroke-linecap:round;stroke-linejoin:round"/><polyline points="0.5 3 0.5 9.5 6 12 8 11.1" style="fill:none;stroke:#949494;stroke-linecap:round;stroke-linejoin:round"/><polyline points="11.5 3 11.5 8" style="fill:none;stroke:#949494;stroke-linecap:round;stroke-linejoin:round"/><line x1="6" y1="5.5" x2="6" y2="12" style="fill:none;stroke:#949494;stroke-linecap:round;stroke-linejoin:round"/><line x1="10" y1="10" x2="15" y2="15" style="fill:none;stroke:#009688;stroke-linecap:round;stroke-linejoin:round"/><line x1="15" y1="10" x2="10" y2="15" style="fill:none;stroke:#009688;stroke-linecap:round;stroke-linejoin:round"/></svg>

resources/images/texture_displacement_select_all.svg

@@ -0,0 +1 @@
+<svg xmlns="http://www.w3.org/2000/svg" width="16" height="16" viewBox="0 0 16 16"><polygon points="0.5 3.5 7.5 6.5 7.5 14.5 0.5 11.5 0.5 3.5" style="fill:#009688;fill-opacity:0.55"/><polygon points="7.5 6.5 14.5 3.5 14.5 11.5 7.5 14.5 7.5 6.5" style="fill:#009688;fill-opacity:0.3"/><polygon points="0.5 3.5 7.5 6.5 14.5 3.5 7.5 0.5 0.5 3.5" style="fill:#009688"/><polygon points="0.5 3.5 7.5 6.5 14.5 3.5 7.5 0.5 0.5 3.5" style="fill:none;stroke:#949494;stroke-linecap:round;stroke-linejoin:round"/><polyline points="0.5 3.5 0.5 11.5 7.5 14.5 14.5 11.5 14.5 3.5" style="fill:none;stroke:#949494;stroke-linecap:round;stroke-linejoin:round"/><line x1="7.5" y1="6.5" x2="7.5" y2="14.5" style="fill:none;stroke:#949494;stroke-linecap:round;stroke-linejoin:round"/></svg>