Bug 1002992. Part 1. Don't bother to do text-overflow clipping when building display lists for image visibility. r=mats

It's not important to do and simplifies things.
Bug 1002992. Part 2. Handle display ports and ignore viewport scrolling for image visibility display lists the same way we do for painting display lists. r=mats
Bug 1002992. Part 3. Don't build the caret when building a display list for image visibility. r=mats

It's not needed and complicates things.
Bug 1002992. Part 4. Factor out the code that gets the presshell to use for painting of subdocuments. r=mats
Bug 1002992. Part 5. Use a custom frame tree walker to find visible and almost visible images instead of using the heavier weight display list infrastructure. r=mats

This is about 3 or 4 times faster.
Bug 1002992. Part 6. Rename ExpandRect to ExpandToNearlyVisibleRect. r=addressingreviewcomment