I forgot to set `BINDLESS_SLOT_COUNT` in `ExtendedMaterial`'s
implementation of `AsBindGroup`, so it didn't actually become bindless.
In fact, it would usually crash with a shader/bind group layout
mismatch, because some parts of Bevy's renderer thought that the
resulting material was bindless while other parts didn't. This commit
corrects the situation.
I had to make `BINDLESS_SLOT_COUNT` a function instead of a constant
because the `ExtendedMaterial` version needs some logic. Unfortunately,
trait methods can't be `const fn`s, so it has to be a runtime function.
3af0b29809