Rhinocommon: Expose TextureType.Environment

Digging through the TextureTypes, I noticed that if I have an environment texture, its ‘TextureType’ is 86, unlike Bump, Bitmap, and Transparency. Though in RenderMaterial I have all of the texture types. Any reason to keep it out of the TextureType Enum?

Here are the comments in openNURBS:

// OBSOLETE - set m_mapping_channel_id = ON_MappingChannel::emap_mapping
emap_texture = 86, // spherical environment mapping.

Perhaps @andy can provide some insight…