Skybox support

[ArthurVoxowl]

Implementation of Sky.Image to load skyboxes:

-- skybox loaded from cubemap file data
Sky.Image = Data:FromBundle("images/skybox.ktx")

-- filtering is ON by default, turn it OFF if using a pixelart skybox
Sky.Image = { Data:FromBundle("images/skybox.ktx"), filtering = false }

-- skybox loaded from a full set of images
Sky.Image = {
   top = Data:FromBundle("images/top.jpg"),
   bottom = Data:FromBundle("images/bottom.jpg"),
   right = Data:FromBundle("images/right.jpg"),
   left = Data:FromBundle("images/left.jpg"),
   front = Data:FromBundle("images/front.jpg"),
   back = Data:FromBundle("images/back.jpg")
}

-- skybox loaded from set of images, with replicated right, left, front, and back
Sky.Image = {
   top = Data:FromBundle("images/top.jpg"),
   bottom = Data:FromBundle("images/bottom.jpg"),
   sides = Data:FromBundle("images/seamlessLandscape.jpg")
}

-- skybox loaded from partial set of images, any missing face will be white (here top and bottom)
Sky.Image = {
   right = Data:FromBundle("images/right.jpg"),
   left = Data:FromBundle("images/left.jpg"),
   front = Data:FromBundle("images/front.jpg"),
   back = Data:FromBundle("images/back.jpg")
}

-- remove cubemap(s)
Sky.Image = nil -- or false

-- skybox colors are applied multiplicatively with cubemap
-- most common use-case is to set them to white, but it can be used to create interesting effects
Sky.SkyColor = Color.White
Sky.HorizonColor = Color.White
Sky.AbyssColor = Color.White

-- preload two cubemaps to lerp later
Sky.Image = {
   cubemap1, -- all options described above to load a cubemap can be used here
   cubemap2,
   t = 0.0
}

-- animate the lerp value later, for example in Client.Tick function
Sky.Image = { t = animatedValue }

[aduermael]

LGTM!