[compositing-1] Normative section

During last week's call [1] David Baron mentioned that the compositing and
blending specification didn't declare the mathematical operations as
normative.
I went back over when I made this change but couldn't retrace why this was
done (or who told me to do so).

I agree with David that this should be declared normative and made changes
to the editor's draft so those section are now normative [2]. I also
updated those sections to clarify what are notes, examples and
required/normative behavior.

Does anyone disagree with this change?
If not, I would like to ask for another Last Call on the next SVG and CSS
telecon with a duration of 4 weeks.

In addition, I also
- removed the paragraph on knockout (per Simon Fraser's request)
- removed the section on clip-to-self and references to it (per James
Robinson's and roc's request)
- clarified section on SVG compositing and ordering of examples and figures
(per Tav's request)


1: http://lists.w3.org/Archives/Public/www-style/2013Dec/0246.html
2: http://dev.w3.org/fxtf/compositing-1/