DirectO: consider adding negative angle parameter

[mtmccrea]

There are some cases in which it's useful to use a negative angle value in DirectO, which has the effect of increasing the perceptual weight of the first order, "directional" components (by reducing the omnidirectional component). Beyond a certain point the coherence of the image breaks down, but it can be useful if used in subtle ways. Currently the slider has a minimum of 0, though you can manually enter negative values and it sounds like it behaves correctly.

If this is deemed useful, somethings to work out are:

• how far into the negative direction to allow (the full -pi/2?)
• what is the visual representation?

It's possible there's another approach to achieving this without changing DirectO. It's essentially just reducing W, which could be done in a channel mixer. But its conceptually an extension of DirectO so may be at home in that plugin.

@joslloand may have some thoughts on this.