Refactor names related to textures (low priority)

[jkbelcher]

Did a refactor pass (name changes only) to clarify a few things:

• iChannel textures
• ModelCoords and ModelIndex textures

There were places where "coords" was used in reference to both model-derived textures, and places where "coords" was used only in reference to the first texture. Attempting to create consistency I chose:

• "ModelTextures" refers to both lxmodel-derived textures
• "Coords" refers to the first texture and "Index" refers to the second texture.

[zranger1]

Looks good! Tested - the iChannel[n] textures and internal coordinate and indexing samplers are all working. I especially appreciate the cleanup of the messy naming around the shadertoy-compatible iChannels. That's been around since before year 1, and it has always been confusing.

Approved, but maybe hold merge 'till after the burn.

[andrewlook]

note: i merged the stacked PR #773 into this branch, then will rebase it into master (to preserve the 3 resulting commits)

[andrewlook]

thanks for thinking ahead here @jkbelcher to make the naming something that is clearer before we keep adding stuff onto this 🙏