[SVG] Filter effects with empty paint rects are valid inputs.

Currently, FilterEffect::createImageBufferResult() refuses to allocate
image buffers for FEs with empty absolute repaint rects - which in turn
causes the filter chain to fail.

But such FEs can still produce results when used as inputs for certain
effects (feComposite for example). We should not break the filter chain
on empty-rect inputs, but let individual FEs sort it out.

BUG=366665
R=senorblanco@chromium.org

Review URL: https://codereview.chromium.org/255783005

git-svn-id: svn://svn.chromium.org/blink/trunk@172660 bbb929c8-8fbe-4397-9dbb-9b2b20218538