Fix issue with coincident geometry + MSAA. See #16899.
101f9eeb changed the polydata fragment shaders to always write out gl_FragDepth explicitly to help with the depth peeling abstractions. However, this causes issues when multisampling, as the multisampled depths are not handled correctly when gl_FragDepth is not the default value.  has some more detailed info. After this patch, gl_FragDepth is only set when it is needed.  http://www.gamedev.net/topic/544920-gl_fragdepth-and-anti-aliasing/#entry4514289
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