Draw capture points behind fleet vessels (#50)

CapturePoint.gd left z_index at its default 0, the same layer Ship
draws on, so a ship parked inside a ring was partly swallowed by the
ring fill - exactly where the player most needs to see their own
vessels. Set z_index = -1, matching the convention every other world
object already follows (Planet, Rock, Building, Hauler, Transporter,
VesselDelivery).

Verified with a ship placed directly on a capture point in the
preview screenshot, and asserted in test_capture_points.gd.

tests/test_capture_points.gd

@@ -197,6 +197,8 @@ func _process(delta: float) -> bool:
 			# through the build UI here to avoid duplicating that coverage.
 			if not game_state.is_planet_conquered(planet_id):
 				return _fail("single-point planet was not marked conquered")
+			if point.z_index >= 0:
+				return _fail("capture point does not draw behind the fleet/enemies (z_index=%d)" % point.z_index)
 
 			print("TEST_CAPTURE_POINTS: PASS")
 			quit(0)