Sky|Renderer: Reimplemented sky renderer with an OO design in C++

Supporting Boom maps requires the ability to draw multiple skies at
once. This commit signals the beginnings of support for this feature.

Todo: The current Map should own the Sky instance(s).
Todo: Use GL2 components for hemisphere geometry construction and
rendering of the sky sphere.
Todo: Cleanup
Merge branch 'master' of github.com:skyjake/Doomsday-Engine