Communicate earlier that the UA doesn't need a depth texture

[cabanier]

With the current implementation, an author can't determine if the UA needs access to the depth texture until after the layer is created.
For implementations that don't need access to the depth buffer this may result in the allocation of a depth texture that will never be used.

It would be great if the session could convey that the depth buffer is not needed so it can skip allocating it.

[cabanier]

/tpac how can we signal that a UA doesn't need a depth texture?

[cabanier]

@toji proposed that this becomes a new attribute on the webgl binding