v0.12.0: Keep a renderer-only name out of the 0.12.0 docs check.

Shaders can read what is already drawn. A lens gets wet over the frame, a
node ripples, and a scene gives way through rising water. All three run
on the CPU, so check and a render without a GPU paint them, and the GPU
renderer draws the same picture.

Added

• shaders::lens_drops() puts drops on the lens over everything drawn
  before it, inside its box. Add it last and make it .fullscreen().
  .wetness() is a signal from 0 (dry) to 1, so
  lens.wetness().to(1.0, 0.4) is a splash. .seed() places the drops and .drop_size() scales
  them. .input("scene", Input::frame()) names what it reads;
  Input::backdrop() is the same outside a layer.
• Node::effect runs a shader on a node's own pixels, children included.
  shaders::ripple() sends rings out from the node's center: .strength()
  and .speed() are signals on the returned Effect. The outline stays,
  so use it on text, a gradient, an image or water.
• Transition::shader paints a transition with a shader preset.
  shaders::water_wipe() raises a wavy waterline over the frame; below it
  is the next scene, rippled near the line. .wave() sets the wave height.
• check reports input_cycle (an error) when a shader would read pixels
  that are not finished: lens_drops or an effect inside a group with
  opacity below 1, a blur, a clip, a blend, a shadow or another effect.
  The shader is skipped there. A misspelled input or lens_drops
  parameter is param_unknown with the right name.
• inspect --graph shows effect passes with the shader that runs, and
  inspect names the preset of each shader node.
• Svg::new().width() and .height() take pct(), read when the node is
  added. A Shader handle converts to Node, so Camera::center_on
  works on water.

Fixed

• Renderers kept every image they had drawn. A waves node paints a new
  image each frame, so a long render grew without bound. They now keep
  only the images of the current frame.
• A shader wider than the safe area (water to the edges) is no longer an
  out_of_frame warning, and lens_drops over text is not covers_text.

Not in this version

Input::node, Input::image and .offscreen() from the roadmap are not
built. A custom WGSL shade is still checked, not run. low_contrast does
not read shader pixels yet.

Breaking

• A renderer of your own needs render_list, which draws a part of the
  draw list. Before → after:

  // before
  fn render(&mut self, graph: &SceneGraph, o: &RenderOptions) -> Image {
      draw(&graph.draw_list(), o)
  }
  // after
  fn render(&mut self, graph: &SceneGraph, o: &RenderOptions) -> Image {
      self.render_list(graph, &graph.draw_list(), o)
  }
  fn render_list(&mut self, _: &SceneGraph, items: &[DrawItem], o: &RenderOptions) -> Image {
      draw(items, o)
  }
  

• The draw list has three markers, ReadFrame, BeginEffect and EndEffect.
  A renderer skips them (Primitive::is_marker on docs.rs); the engine
  handles them.

• The pass and inspect node structs have a new shader field.