add and update tests for linear interpolation bleeding

This adds a `linear-filter-opacity-edge` that tests that the rgb values of a fully transparent pixel don't get interpolated with neighbouring values of visible pixels.

The existing tests all cover specific style properties while this covers something that affects all icons and fill patterns. Is it ok to have this separate, or should this be made part of some property's tests?

This also updates all images for tests that were affected.

@jfirebaugh