Sampler and Filter Updates

Sampling:

* Added the boundingMode parameter to GafferImage::Sampler() which defines how samples that are out of bounds are handled.
      - Current options are: Black, Clamp.
* Added bindings for the different bounding modes.
* Added test cases for sampling with the different bounding modes.
* Added sub-pixel sampling.
* Fixed a typo in the bindings.
* Added test cases.

Filters:
* Reordered the list of filters to most blurry -> sharpest
* Added the Lanczos filter.
* Fixed the Sinc filter.
* Fixed the Mitchell filter.
* Updated the test cases and the images.

Fixed Issues:
* Fixed the "Streaking" seen at the edges of the ImageTransform node.