Blender Git Loki

3 very simple new composite nodes that I wanted for working with here. Hope the code is ok,
they work ok in testing here and get done what I need, any checks or fixes are welcome.

* Separate RGBA: Separates an input RGBA image into its R, G, B and A channels
* Separate HSVA: Separates an input RGBA image into H, S, V and A channels
* Set Alpha: Takes an input RGBA image and an alpha value channel and combines them
into a single RGBA image channel. You can also set the alpha for the entire image
with the number field when there's no input alpha channel. TODO: Allow input alpha
channel with no input image, in order to output a solid colour, with alpha.