Re-enable new planets

Allow loading of cube map data, cube maps now work correctly in non-shader mode.
Cube map support for new planets
Fix last two cherry-picked commits so that they compile again

The whole NEW_PLANETS bit requires a fairly decent graphics card, so it's basically turned off unless the detail level is at "extras".
Draw atmosphere, and generate it if texture was not generated.
Try new atmosphere
Make NEW_ATMOSPHERE clouds a bit more visible
Regenerate planet texture if detail level changed
Add a missing _pendingOpsLock pair to the Async manager

Also, since we're hitting the bug anyway, re-enable planet preloading - it might make it easier to see what's going on
Fix probably theoretical bug
Try to work around bugs in pthread library on Windows
Merge pull request #75 from OoliteProject/planet-improvements

Planet improvements