GafferSeed

appleseed (http://appleseedhq.net/) support for Gaffer.
Works with appleseed's master branch.

Minimal instructions:
---------------------------

- $APPLESEED must be defined and point to appleseed's directory.
- appleseed's binary dir must be on the $PATH.
- appleseed's lib dir must be on the linker path.
- appleseed's python module is only needed to run IECoreAppleseed tests (inside Cortex).
- Uses OSL shaders. They are found using the $APPLESEED_SEARCHPATH env var.
- There's a basic OSL shaders lib (work in progress) for appleseed that can be used in Gaffer here: https://github.com/appleseedhq/appleseed.shaders.
- Only supports rgb and rgba outputs, interactive, batch/exr and batch/png.