View source for Gecko:Frame Painting

Currently our handling of stacking contexts and z-ordering is deficient because nsIFrame::Paint can't separate the painting of a frame's background from the painting of its children. See [http://bugzilla.mozilla.org/show_bug.cgi?id=78087 bug #78087].

We also have some problems because currently a frame with a view paints all its descendant frames-without-views in one go. Sometimes that's not correct. e.g.:
<pre><nowiki>
     F (V) 
      / \\  

  F1(V1)  F2 
</nowiki></pre>
we'll build a display list "V V1"; painting V paints F and F2. So the rendered order is F F2 F1, which is wrong.

I think the right approach is to move all the display list stuff out of the view system and into frames. Display list elements will be instructions like "paint border of frame X", "paint background of frame Y", "paint text in frame Z" and whatever other paint commands are required by various frame types. Elements would only be needed for frames that actually have borders, backgrounds etc. (Except that for event handling there should always be a background element
to catch events for the frame.)

The frame display list can be built by with code similar to [ http://www.w3.org/TR/CSS21/zindex.html]. We should be able to avoid of the trickery of display list construction in the view system.

One question is whether we really need a display list if we could paint everything in one recursive traversal. Given that it lets us unify event handling, bitblit analysis, and painting, and also lets us optimize cases where lower frames are covered by opaque higher ones, and serves the needs of various embedders, I think it's worth having. Also I think that we can construct it in just one traversal of the frame tree, which could be more effecient than doing several passes over the frame tree if we directly followed Hixie's z-order spec.  (E.g., doing separate passes over large frame subtrees just to see if there are any 'outline's seems very slow and wasteful). Also we can do some useful postprocessing of the display list such as outline merging.