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.

added Rendering label

marked this issue as related to #5987 (closed)

marked this issue as related to #3865

marked this issue as related to #5088 (closed)

assigned to @glassmancody.info

mentioned in issue #5492 (closed)

mentioned in merge request !1057 (merged)

mentioned in issue #6213

closed

changed milestone to %openmw-0.48

mentioned in merge request !1124 (merged)

added Task label

changed title from Refactor Sky to use shaders and GLES/GL3 friendly to Refactor Sky to use shaders and be GLES/GL3 friendly