`unfiterable : true` indicates that the sampling of the texture
will not apply filtering.
This is mainly to satisfy the webgpu requirement for
bindGroupLayouts.
- Documentation has been updated for both `unfilterable` and
`multisample`.
- Internal materials have been updated where necessary (depth
samplers have to be marked as `unfilterable`.)
- webgpu code has been changed appropriately.
BUGS=420745987
f096c4d8de
