Create transfer color textures Map and use a single texture unit

As we dont update dynamically the transfer color textures at runtime, we can create a texture map at loading time, and create a single transfer texture map. The volumes only index the color. This way we can send more volume models and use only one for the transfer textures.

Start by locating where transfer color textures are created during loading and where volumes index their colors. Trace the texture-unit and shader setup to understand the current bindings. Done means transfer textures are represented by one map, volumes index its colors, and multiple volume models can use it without runtime texture updates.

Written by the indexing model from the issue text.