Use correct destination rectangle in FEComposite w/ type='in'

When one of the inputs are larger than the other, the destination will
assume the size of the smaller. When drawing the input onto the
destination however, the size of the buffer itself is always used. This
means that the source will be stretched onto a (too large) destination
rectangle. Destination and sources should always be the same dimensions
in this particular case, so make sure they are by simply adjusting the
computed destination rectangle based by the effects paint rect, hence
retaining the computed dimensions.

BUG=391200

Review URL: https://codereview.chromium.org/365163002

git-svn-id: svn://svn.chromium.org/blink/trunk@177536 bbb929c8-8fbe-4397-9dbb-9b2b20218538