Scaling and offset fix for FELighting (software and skia paths)

This fixes scaling (including z-scaling) caused by page zoom and high dpi settings (pinch zoom still todo).

This also fixes the light position, since the light origin was misplaced when a non 0 filter effect region x and y parameters were used (which is the case by default). The origin was affected by these values, which shouldn't be the case, as these provide extra space to render an effect, like a blur, for example, but should not changed the effect's coordinate system.

BUG=

Review URL: https://codereview.chromium.org/181943003

git-svn-id: svn://svn.chromium.org/blink/trunk@168526 bbb929c8-8fbe-4397-9dbb-9b2b20218538