Added a MapOffset node for offsetting texture coordinates.

The offset is specified as a combination of a V2f and a UDIM tile number.
Made Plug::acceptsInput() consider current output connections.

A plug will now never accept an input if any of its immediate outputs would not themselves accept the same connection directly. This allows the RenderManShader node to accept connections to Boxes without them being used a loophole to bypass the usual connection checks for coshaders.

Fixes #532.
Specified specific version for default Cortex source.

Before, the public build was being made with some unspecified Cortex version straight off the master. Now it will be made with a specific released Cortex version, so others can reproduce the same build.
Fixed GraphGadget for NULL return from NodeGadget::create().

This allows NodeGadget::registerNodeGadget() to be used with functions that will return NULL to signify that the node should be hidden.
Merge pull request #552 from johnhaddon/acceptsInputConsideringOutputs

Made Plug::acceptsInput() consider current output connections.
Merge pull request #553 from johnhaddon/nodeGadgetCreatorNullReturn

Fixed GraphGadget for NULL return from NodeGadget::create().
Merge pull request #554 from johnhaddon/mapOffset

Added a MapOffset node for offsetting texture coordinates.
Bumped version to 0.82.0 and updated Changes file.