Resolve light-source position/direction when updated

When 'x'/'y'/'z' on fePointLight/feSpotLight, or
'pointsAtX'/'pointsAtY'/'pointsAtZ' on feSpotLight is mutated (animated,
changed through the DOM) and filter primitive units is 'objectBoundingBox',
the new coordinate needs to be resolved against the filter's bounding-box.
Also simplify getting these position/direction attributes of a light-source
element by adding new methods on SVGFELightElement.

BUG=393907

Review URL: https://codereview.chromium.org/395813002

git-svn-id: svn://svn.chromium.org/blink/trunk@178167 bbb929c8-8fbe-4397-9dbb-9b2b20218538