[css-compositing] Additional blend modes

We've been getting many requests from designers and web developers to add
Photoshop's blend modes to the current working draft.
They felt that these blending modes are commonly used and are needed to
achieve certain effects.

I propose that we extend the current list of blending modes with these
values:

Linear dodge
Linear burn
Vivid light
Linear light
Pin light
Hard mix
Divide
Subtract

These blend modes are not in the PDF spec. I checked the open source
browsers (and Apple's CoreGraphics) and they don't have an implementation
for them which may slow down their adoption.
If there are no objections, I will update the spec with the blend formulas
and example bitmaps.

Rik Cabanier