Avoid concurrently overflowing of shape_id

Previously it was possible for two atomic increments of next_shape_id
running concurrently to overflow MAX_SHAPE_ID. For this reason we need
to do the test atomically with the allocation in shape_alloc returning
NULL.

This avoids overflowing next_shape_id by repeatedly attempting a CAS.
Alternatively we could have allowed incrementing past MAX_SHAPE_ID and
then clamping in rb_shapes_count, but this seems simpler.

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