v0.19.0: Release 0.19.0: 3D scenes inside 2D ones.

3D scenes inside 2D ones, behind the 3d feature.

Added

• Scene3d is a world of meshes: Mesh::cube(), Mesh::sphere(),
  Mesh::plane(), Mesh::cylinder(), Mesh::group() and models from a
  binary glTF (Mesh::glb_file). .color(), .metallic(),
  .roughness(), .emissive() and .opacity() set the material.
• scene.add(mesh) returns an Object3d whose position, rotation,
  scale, color and opacity are signals, so .to() animates them;
  object.add(mesh) makes a child that moves with its parent.
• scene.camera() (position, target, field of view) and
  scene.light(Light::directional(..)) or Light::point(..) are signals
  too; with no light, a default sunlight shades each face differently.
• View3d::new(&scene).size(640.0, 400.0) shows the scene as a 2D node:
  position, opacity, effects, the 2D camera and layout work as for any
  node. view.projector() maps a 3D point into the view, so a 2D label
  follows a corner of a model.
• The renderer is a software rasterizer with a depth buffer and 2 × 2
  supersampling: the same scene gives the same pixels in a render, in
  check, in CI without a GPU and in the browser playground.
• DrawCtx::image draws a picture from a node of your own.
• The cube_3d example.