Fill holes at the poles

[pjcozzi]

Some ideas

1. Do it in screen space like how we fill cracks. Scissor out the projected bounding rectangle of the bounding sphere for the hole. Ray cast or stencil the ellipsoid, or use the alpha channel. When shading, still use the geodetic ellipsoid surface normal.
2. Render a low-resolution globe under the actual globe. Render it second for early-z.
3. Render low-resolution geometry for just the poles under-lapped a bit to avoid cracking.
4. Create tiles with a 1x1 white texture

I suggest 1. It's fragment load is higher than the others, but I expect the visual quality to be the best.

[bagnell]

I chose to implement the screen space solution. See pull request #42.