[Graphics] Direct3D 12: missing support for generating mipmaps on 3D render textures

1. What happened

We use 3D (volume) render textures with runtime-generated mipmaps in our game, and while investigating a graphical corruption issue on Direct3D 12 I noticed the D3D12 backend doesn't seem to have support for GenerateMipmaps with 3D render textures (or array textures either).

Our game has dynamic volumetric clouds which are generated (and can be user-configured, if a user creates their own level) at runtime. Some of the volumetric information is calculated and stored in mipmapped 3D render textures by using a compute shader to write to the first mipmap level and then calling GenerateMips after the compute dispatch.