v0.8.0: Describe a frame as passes and reuse a layer that did not change.

A frame's draw list is a pass graph, and a layer whose vectors did not
change is reused (ROADMAP task 15, stage 2). No new scene API and no shaders.

Added

• Render passes in topological order: one vector pass when nothing opens a
  layer, then a composite pass for a group opacity, a blend, a blur, a shadow
  or a clip. Contents run before the composite that uses them.
• A static layer is cached as the pixmap from before composite, so the next
  frame skips redrawing that layer. The root vector pass is not hashed, and a
  layer that moved or changed is drawn again. CPU bytes stay the reference.
• inspect --graph adds a passes array (kind, node, name, items,
  depth, opacity, blend, filtered). Without the flag the JSON is
  unchanged.