[icelab] seamless skybox

Seamless skybox with floor.
The sky part is mesh only, the floor part contains mesh and transparent prim.

When the sky part is unlinked from the floor part, it is possible to rez objects to the floor even if camera position is outside of the skybox. Not works when everything is linked together.

Land impact: 16 for 32x32 or smaller size, 35 for 64x64 size.

Texture changer is included, only works inside the skybox.
You can choose between 2 textures: forest or beach.