Add Luma Key filter

This commit adds a luma key filter that can be used to composite images based on source image's luma value.  Parameters can be used to set threshold, direction and sharpness of opacity transition, and over all opacity values before and after luma threshold.

The test render file for this filter linked below gives an example of an effect that would hard to achieve without a luma key filter with post- and preopacity level controls.

https://dl.dropboxusercontent.com/u/18599079/lumaHD.mpg