Corrections

Here are various corrections to take into account the implementations of Krita, Gimp, Pinta and MyPaint.

• formatting and links in the specs
• x and y attributes are ignored for stacks
• default value for opacity
• it is not a draft anymore but is kind of stable
• "isolation=auto" is never specified except by Krita and in this case, the meaning of "auto" is to have a non-isolate stack even when there are non trivial blend modes or opacity. On the other side, what was said in the documentation would mean that the "isolation" field would make no difference. So I changed the description to say that "isolate" is the default value of the property and that "auto" means to use non-isolation in all cases.
• update of the RNC and RNG schemes