Fix bug in SpanSelector, introduced in commit #dd325759

The useblit attribute was being set after calling new_axes,
but new_axes needs that attribute.  By setting useblit before
calling new_axes, we set it based on the initial canvas; even
though new_axes might change to a different canvas, we assume
it will be the same type of canvas (same backend), so its
supports_blit attribute will not change.