v0.4.0

Reference images: trace voxels over loaded pixel art, drawn in the viewport as a
flat, depth-tested guide on roxlap 0.12's world-placed image sprites.

Added

• Reference images: load pixel art (PNG / BMP / JPG / GIF / TGA / WEBP) as a
  flat guide to trace voxels from — via File ▸ Open reference image or by
  dragging an image onto the window. It's non-destructive (never
  saved/exported/edited): place it on the Front / Side / Top plane, set its
  depth, flip it, hide it, or remove it. The Reference panel's Move toggle
  lets you drag it into position on the grid with the mouse (left-drag slides it
  in its plane, whole-voxel snap). It's drawn as a flat, world-placed image
  sprite (roxlap 0.12 draw_images), so the model occludes the parts behind it
  and it stays undistorted from any angle, with an Opacity slider to dim a
  too-bright reference to a faint guide. The eyedropper (the tool, or Ctrl
  +click from any tool) picks colours straight off the reference image —
  whichever of the model voxel or the reference is nearer the cursor wins.
  Dropping a .kv6 / .vox / .demiurg file opens it as the model. The tool
  panel now scrolls so every section stays reachable.

Changed

• Bumped roxlap to 0.12.0, for the world-placed 2D image sprites
  (SceneRenderer::upload_image / draw_images) that draw reference images.