Add KHR_TF_2D extension to support 2D Transfer Functions for Volumes about anari-docs HOT 1 OPEN

Simple case (Implicit 2D transfer function for scalar data)

1. SpatialField stays the same (single scalar value)
2. Add gradient parameter to scivis volumes
3. Add tf2D parameter to scivis volumes of type Sampler with subtype image2D for explicitly defining the 2D transfer function look-up table with the second dimension representing the gradient magnitude

More general case (Explicit and implicit 2D transfer function for multivariate data)

1. SpatialField supports up to vec2 scalar values
2. Add second opacity parameter to scivis volumes that maps the second scalar value to an opacity multiplier
3. Add tf2D parameter to scivis volumes of type Sampler with subtype image2D for explicitly defining the 2D transfer function look-up table

from anari-docs.