RenderListWheelViewport class

Render, onto a wheel, a bigger sequential set of objects inside this viewport.

Takes a scrollable set of fixed sized RenderBoxes and renders them sequentially from top down on a vertical scrolling axis.

It starts with the first scrollable item in the center of the main axis and ends with the last scrollable item in the center of the main axis. This is in contrast to typical lists that start with the first scrollable item at the start of the main axis and ends with the last scrollable item at the end of the main axis.

Instead of rendering its children on a flat plane, it renders them as if each child is broken into its own plane and that plane is perpendicularly fixed onto a cylinder which rotates along the scrolling axis.

This class works in 3 coordinate systems:

1. The scrollable layout coordinates. This coordinate system is used to communicate with ViewportOffset and describes its children's abstract offset from the beginning of the scrollable list at (0.0, 0.0).

   The list is scrollable from the start of the first child item to the start of the last child item.

   Children's layout coordinates don't change as the viewport scrolls.

2. The untransformed plane's viewport painting coordinates. Children are not painted in this coordinate system. It's an abstract intermediary used before transforming into the next cylindrical coordinate system.

   This system is the scrollable layout coordinates translated by the scroll offset such that (0.0, 0.0) is the top left corner of the viewport.

   Because the viewport is centered at the scrollable list's scroll offset instead of starting at the scroll offset, there are paintable children ~1/2 viewport length before and after the scroll offset instead of ~1 viewport length after the scroll offset.

   Children's visibility inclusion in the viewport is determined in this system regardless of the cylinder's properties such as diameterRatio or perspective. In other words, a 100px long viewport will always paint 10-11 visible 10px children if there are enough children in the viewport.

3. The transformed cylindrical space viewport painting coordinates. Children from system 2 get their positions transformed into a cylindrical projection matrix instead of its cartesian offset with respect to the scroll offset.

   Children in this coordinate system are painted.

   The wheel's size and the maximum and minimum visible angles are both controlled by diameterRatio. Children visible in the untransformed plane's viewport painting coordinates's viewport will be radially evenly laid out between the maximum and minimum angles determined by intersecting the viewport's main axis length with a cylinder whose diameter is diameterRatio times longer, as long as those angles are between -pi/2 and pi/2.

   For example, if diameterRatio is 2.0 and this RenderListWheelViewport is 100.0px in the main axis, then the diameter is 200.0. And children will be evenly laid out between that cylinder's -arcsin(1/2) and arcsin(1/2) angles.

   The cylinder's 0 degree side is always centered in the RenderListWheelViewport. The transformation from untransformed plane's viewport painting coordinates is also done such that the child in the center of that plane will be mostly untransformed with children above and below it being transformed more as the angle increases.