New drawing/scrolling model landed

[Alexander Larsson <alexl redhat com>]

I just merged the wip/simple-draw4 branch with master. It seems to work
pretty well now. Open issues i know of:

Anything using GdStack to do crossfades will show some weird coloring
during the fade, these apps should upgrade to the latest libgd (or
ideally to GtkStack).

The gnome-cc background panel uses gdk_cairo_create() rather than use
the passed in cairo_t in draw(), so it looks weird in the crossfade.

There is some things left to do for this, some we really need to do
before 3.10 is released:

* Make GtkIconView use GtkPixelCache
* Make GtkTextView use GtkPixelCache

Some are optional but may be interesting:

* Allow some kind of hinting to GtkPixelCache so that e.g. we always
  fit full rows in a GtkTreeview. This is nice as there is some
  per-row drawing overhead which we then avoid when scrolling.

* GtkViewport generally always has a transparent background, but it
  often happens that the child does not. Atm this means that we render
  the pixel cache as RGBA starting at transparent and then composite
  it using OVER. However, in the case of a child with a non-transparent
  background (like a white EggListBox) we really would want to know that
  the child is opaque so that we can just use a RGB buffer and SOURCE
  rendering.