rtx: advertise and rename VISRTX_MATERIAL_MDL extension to VISRTX_MATERIAL_MDL_SURFACE

[tarcila]

Frees the bare MDL name for the material subtype namespace and documents the extension in the device README.

[jeffamstutz]

I approve that we should report the extension, but I don't think it should be called MDL_SURFACE, instead we should keep the name just MDL (as VISRTX_MATERIAL_MDL). The _SURFACE suffix is redundant because if we bring MDL to volumes it will be under a separate concept -- we would name that VISRTX_VOLUME_MDL accordingly.

While some physically based renderers like to treat volumes as-if they are materials (a fine taxonomy to use within VisRTX, if we so choose), the ANARI standard explicitly avoids this crossover by making "material" a strictly surface concept.

[jeffamstutz]

Frees the bare MDL name for the material subtype namespace

I suppose I did leap to a conclusion that volumes were what we are trying to reserve as a namespace. Are there other ANARI_MATERIAL variants that will use MDL which are different than this one?

[tarcila]

Frees the bare MDL name for the material subtype namespace

I suppose I did leap to a conclusion that volumes were what we are trying to reserve as a namespace. Are there other ANARI_MATERIAL variants that will use MDL which are different than this one?

And you did leap to the right conclusion. The intent is both to be more explicit about the restrictions that the MDL support has, that is no volume, and to keep the door opened to an actual volume support.
That being said, as you pointed out, from an ANARI standpoint, material and volume are different concepts.
I'll drop the renaming keep the exposure.