Fix Labels with backgrounds

[bagnell]

Revert to old billboard rendering method when labels have a background. Fixes #4927.

[emackey]

Take a look at the Everest Terrain demo in Sandcastle, click Sample Everest Terrain (on the 3rd row) to see lots of labels. If you try this in Cesium 1.29, it will look a lot better than what's on this branch or in master. The old mode doesn't exactly correspond to typical OPAQUE or TRANSPARENT, as it had blending turned on but was also writing depth for non-zero alpha pixels, meaning it was expecting the labels to be "mostly" opaque, like a cutout mode. Of course this strategy punched holes in things at the alpha fringes, but when label backgrounds are on there's no fringe. Is it worth trying to get this mode back? I think so...

[bagnell]

Thanks @emackey. Updated.

[pjcozzi]

Code, tests, and related Sandcastle examples look good.

Merging since this is needed for the release tomorrow morning.

@bagnell thanks for the fix.

@emackey if you have more feedback, please submit an issue.