extract skybox with wrapping safety

[guanyilun]

Extracting a skybox from a map needs to be safe when wrapping is involved.

[xzackli]

The most common situation involves wrapping over RA, so that can be the first thing to support. Handling the poles is slightly more involved.

The way we've chosen dimensions, RA is the first dimension and thus rings have stride=1 (good for SHTs). That means that a fast extraction should be something like

1. identify where the RA seam is, by doing sky2pix on each RA of the x-axis
2. generate two slices corresponding to "left of seam" and "right of seam".
3. copy over those two pieces