Clean-up renderImage paintIntoRect, add FIXME

Calculate alignedRect when needed, image.get() is inline so use that
(removes unnecessary temporary), add FIXME.

No change in behavior, no new tests.

BUG=430772

Review URL: https://codereview.chromium.org/703073003

git-svn-id: svn://svn.chromium.org/blink/trunk@184940 bbb929c8-8fbe-4397-9dbb-9b2b20218538