Fix Metal shader compile issue

[pendevere]

Passing a read_write storage texture (like clouds_render_texture) as an arg to a helper function can cause permission confusions in Metal. By moving into main function, compiler can easily see that the global variable has read_write permissions.

To remain in scope we must change from texture to clouds_render_texture.

Tested on: Metal, Windows Vulkan

[evroon]

Thanks!