Don't leak vectors freeing target

Signed-off-by: Casey Dahlin <cdahlin@redhat.com>
Introduce target_draw, and target steps

A target can now execute a sequence of draw operations in order when hit.

This is a step toward making a target a semi-sequential thing. We can reorder
things within it implicitly, and we should be able to detect dependencies.

Signed-off-by: Casey Dahlin <cdahlin@redhat.com>