Create "Rect" shape renderer with triangle-tab geometry

[zachkinstner]

The goal is to have a built-in renderer that can support tab graphics like the ones in "Firework Factory VR":

[zachkinstner]

The new renderer is available via the HoverAlphaButtonRectRenderer-Tab prefab. It includes a couple new components for building the tabbed mesh, updating the meshes to have consistent tab sizes, etc.

The tab renderer is meant for use with either Radio or Checkbox items. The HoverAlphaFillTabUpdater component (quite a name, I know...) on the Fill object is responsible for updating all the meshes, including setting them to an alternate color (and keeping the "edge" visible) when the radio/checkbox is selected. The tabs use this approach by default, rather than showing a radio/checkbox icon.

This animation is from the RectTabRowLayout scene in "HoverDemos":

The renderer can support any combination of N/S/E/W tab triangles. The tab "outward" distance and "thickness" distance (perpendicular to "outward") are configurable, also. Here's an example with all four tab directions enabled (with the GameObject selected to show the mesh structure):