Use async texture creation for encoded metadata tables

[nithinp7]

Only applicable once #975 is merged.

We currently use the "legacy" texture creation code path to create encoded metadata textures. This requires RHI texture creation on the render thread, which can impact the framerate. In #975 we added a new texture creation path to offload regular glTF and raster texture creation to background threads - we should take advantage of this for encoded metadata textures as well.