[WIP] initial skybox commit, uses image from vfs in "cross" format

[psi29a]

, 600x800 for example: s:600, t800, one_third_s:200, two_thirds_s:400, one_quarter_t:200, one_half_t:400 three_quarters_t:600 -> has to be squares

[psi29a]

WIP, doesn't quote work yet. Image is accepted, but the skybox only renders one colour, no texture.