demos: make alienplanet demo work on macOS OpenGL

On the macOS OpenGL implementation, the use of noise2 as a function within the glsl shader collides with the builtin noise2 of a different signature.

This changes the name to something similar (noize2) so that we do not risk colliding names when linking.

With this commit, the shadertoy alienplanet demo works on mac OpenGL (albeit still with the Cairo renderer).