Fixed threading issue in GafferImage::Display node.

Access to the tiles in the display driver is now protected by a mutex.
Improved StandardGraphLayout node auto-positioning.

It used assume that the tangent at each end of a connection were opposing - basically assuming that all connections were either horizontal or vertical. By using both endpoint tangents we can now cope with vertical->horizontal and horizontal->vertical connections.
Moved scene node filter plugs to the right hand side of the node.

This makes for tidier graphs.
Merge pull request #716 from johnhaddon/filterPlugPosition

Filter plug position
Merge pull request #714 from johnhaddon/displayThreading

Fixed threading issue in GafferImage::Display node.