Non-visible subset layers shouldn't have textures

[astrofrog]

Non-visible data layers need to have textures if they are used by a subset (for the 'data' mode) but non-visible subset layers definitely don't need to have textures, so we should modify the shader-generating code to take this into account. This will help alleviate the issue of 'no free slots' when adding too many layers to the 3D viewer.