Refactor Sky to use shaders and be GLES/GL3 friendly

This is a task that helps move OpenMW in the direction of being able to be fully take advantage of GL3+ and GLES OSG renderers.

How we handle our Sky (sky box, dome, weather/clouds, moons) was one of the first systems we worked on and we have mostly left it alone. There are still issues with Sky on some systems, so after this task is finished, we should reevaluate the other tickets to see if they were fixed along the way.