GtkPixelCache is a helper utility that lets you implement
faster scrolling of a viewport of a canvas by using an
offscreen pixmap cache.

You call _gtk_pixel_cache_draw with a callback function that
does the drawing, and additionally you specify the size and the
position of the viewport in the widget, and the position and size
of the canvas wrt the viewport. The callback will be called to
draw on an offscreen surface, and the surface will be drawn
on the window. The next time you do the same, any already drawn
pieces of the surface are re-used from the offscreen and need
not be rendered again.

If things inside the canvas change you need to call
_gtk_pixel_cache_invalidate to tell the cache about this.

Some other details:

* The offscreen surface is generally a bit larger than
  the viewport, so scrolling a small amount can often
  be done without redrawing children.
* If the canvas is not larger than the viewport no
  offscreen surface is used.

GtkPixelCache: Make sure we always copy using SOURCE

We were using OVER for the first copy (from source to group surface.

GtkPixelCache: Fix x/y typos

GtkPixelCache: Allow NULL for invalidate region

gtkpixelcache: Use CONTENT_COLOR for solid bg windows