Side view shows wrong clip positions when near plane is behind far plane

[olibclarke@…]

Oliver Clarke accidentally put the near clip plane behind the far clip plane using a command and was confused because the graphics showed nothing but the side view looked as if the planes were correctly placed. While the graphics allows setting near / far to these weird values it clamps the actual clip plane positions so near is at or in front of far. Side view should display the positions after the clamping.

This is a pretty minor and obscure problem. The clip command now warns when you reverse near/far.

Original ticket was #1646.