image3d vex context

Write a program for use with the i3dgen program to generate 3D textures.

The Image3D context is used by the stand-alone program i3dgen to generate 3D texture images. In turn, these 3D texture images may be used by the texture3d() function calls in VEX to efficiently evaluate the 3D texture images.

Any export variables in the image3d context will cause additional channels to be created in the 3D texture map.