Add ability to use same buffer in multiple bindings

[Ilkka Saarelainen]

Buffer descriptor offsets and ranges are now supported, so it makes sense to bind one buffer to multiple bindings.
For example the following is not allowed currently:

BIND BUFFER buf0 AS storage DESCRIPTOR_SET 0 BINDING 0
BIND BUFFER buf0 AS storage DESCRIPTOR_SET 0 BINDING 1 DESCRIPTOR_OFFSET 256