This feature should not be shipped until the various definitions of addition for
each additive property are properly specified and then implemented accordingly.
Unlike other patches in this series, compositing is not frequently used
internally so there is no need to enable this by default for chrome callers.
Since these are just interpreted comments, there's 0 impact on actual code.
This removes all lines that match /* vim: set(.*)tw=80: */ with S&R -- there are
a few others scattered around which will be removed manually in a second part.
KeyFrameEffect and KeyFrameEffectReadOnly are still built unified as
buddies because they have some complex interdependency that couldn't
easily be resolved without linking failures.