[FEATURE] Allow MAPINFO "sky2" and "doublesky" commands

[thefatwilly]

Is your feature request related to a problem? Please describe.
Odamex to allow the mapinfo variables of "sky2" and "doublesky" to give mappers that want to have loads of variety in sky textures to be able to save some space in a wad by utilising the ability to 'graft' together sky textures in the MAPINFO lump. I.E. large map compilations and various CTF projects (like corectf) could seriously benefit from this and make downloading chunky wad files a little less tedious.

[rakohus]

@thefatwilly Could you provide some example wads that use these features?

[thefatwilly]

This is apparently a rare zdoom feature to be used, so I will have to generate a wad to show. Otherwise most mappers simply use the skybox feature.