[filter-effects] proposal: currentColor in feColorMatrix

I would like to create a filter similar to the grayscale and sepia canned
effects, but using currentColor.

To do this, it would seem I would need to somehow use the currentColor CSS
variable in the feColorMatrix describing the filter.

I may be missing something, but I don't see how I could do that, using the
current spec.

The need for this feature is described in more detail here:

http://dencklatronic.blogspot.com/2014/09/the-web-needs-bitonal-images.html

Thank you.