Improve env_sky2d

[Admer456]

env_sky2d has the following two issues:

• it supports only up to 128x128 textures
• it renders in front of transparent surfaces

One way to solve the first is to render more quads in TriAPI to support 256x256 textures, and thus, eventually 512x512. However, that's a problem because we'd have to render more quads and do more maths, and that's going to slow things down. Ideally, we would use an image-loading library and OpenGL code to render the skybox, and it is how I'm intending to do it.

As for the second, I'm not entirely sure.

Since solving this properly will require some custom OpenGL code, this issue is aimed at 0.2.0 instead of 0.1.0