Trigger automatic redirection updates on GetImage and SourceValidate (v3)

[ville.syrjala at nokia.com]

Updated the spec in patch 2/7.

Settled for the compPaintWindowToParent and compPaintChildrenToWindow
names for the window update functions in patch 6/7.

Added patch 7/7. Not sure if it's really worth the extra code lines,
but at least it should allow accelerated copies from backing store for
non-EXA drivers.

And regarding the unwrapping order for SourceValidate; It doesn't
really matter since we paint the children before the parent. So
recursing into compSourceValidate from compWindowUpdateAutomatic
wouldn't do anything since damagedDescendants would already be
cleared for the window by compPaintChildrenToWindow. And this
actually happens if the update process was triggered by compGetImage
or compBlockHandler since those don't unwrap SourceValidate. So I
could change all of these wrappers to do the unwrapping in the more
typical order, if that's preferred.