Agg snapping fixes (for the last time...?) :)

Fix a number of snapping issues.  Incorporates ideas from #800 (see there for further discussion) and #1591.  This, unlike #800, doesn't change the snapping in a fundamental way, but it does help with text/image/marker alignment of snapped and non-snapped objects.

This, unfortunately necessarily, updates all of the PNG baseline images.

We should consider whether we want this on v1.2.x or on master -- on some level it's a bug fix by making elements line up better.  On another level, it's changing behavior, albeit at the subpixel level.