[HLSL] Implement Texture2D::mips[][] (#186143)
We implement the Textur2D::mips[][] method. We follow the design in DXC.
There is a new member called `mips` with type mips_type. The member will
contain a copy of the handle for the texture.
The type `mips_type` will have a member function `operator[]` that takes
a level, and returns a `mips_slice_type`. The slice will contain the
handle and the level. It also has an operator[] member function that
take a coordinate. It will do a load from the handle with the level and
coordinate, and return that value.
Assisted-by: Gemini
