Standardize naming around texture/labels/IDs

[russelldj]

There's a couple words floating around relating to the idea of adding a texture, label, or ID to a mesh, image, or 2D polygon. We should try to standardize the language around this for interpretability.

In my mind, texture is the broadest word. In most cases, it does refer to a scalar or integer label, so maybe label makes sense in most contexts. The one exception is colors. The word ID I think should be avoided if there's a chance we're actually talking about scalar/continuous data. But I think it's reasonable in contexts where we are explicitly processing integer class ID data.

[russelldj]

We should probably also include some term for human-readable labels in this discussion. Maybe "classes" is a good term.