The bind group in set_bind_group should be optional

[nical]

Spec: https://gpuweb.github.io/gpuweb/#gpubindingcommandsmixin-setbindgroup

The bind group is nullable which in rust parlance means we should take an Option<BindGroupId> and be able to "unset" a bindgroup at an index.

[jimblandy]

It's unclear what the underlying platform APIs' expectations are around removing bindgroups, so we're going to need to investigate this a bit before we can tell how much work it will be.

[teoxoy]

Relevant spec PR: gpuweb/gpuweb#3938

[Wumpf]

this got fixed in

• Make bind group an Option for set_bind_group calls. #6216

[Wumpf]

ah I spoke too soon, HAL doesn't handle this yet it seems?