feature: shape components

[reromanlee]

Wireframes (Unreleased)

What's new

1. Shape components draw a shape on their GameObject without code, in Edit Mode, Play Mode and builds: Add Component > Wireframes has one for every shape, such as WireframeSphere and WireframeLine.
2. Every change to a component shows up in the next render, made in the Inspector, by undo, animation or a script, and components do no work per frame.
3. Disabling a component or its GameObject hides its shape at no cost, and enabling it again allocates nothing.
4. Components share containers by occlusion, transparency and layer, so many of them draw in a few draw calls; a color with alpha below 1 draws transparent, and shapes draw on their GameObject's layer.
5. DrawAsGizmo draws a component's shape like a gizmo, in the Scene view and in the Game view while its Gizmos button is on, and builds leave it out.
6. Prefab Mode draws the components of the prefab being edited.
7. The Shape Gallery sample has a ComponentGallery scene made of components, and continuous integration builds it too.
8. A container without shapes skips its work before each render once its empty chunks are released.

Known issues

1. The Hierarchy's Scene visibility toggles don't hide components' wireframes, and the Gizmos menu's per-component checkboxes don't affect gizmo shapes.
2. Clicking a component's wireframe in the Scene view doesn't select its GameObject.
3. A script that changes a GameObject's layer moves its components' shapes to that layer on their next change.
4. Stereo rendering for XR is untested.