render: fix connection label mask position

[gavin-ts]

Summary

Parts of connections could become hidden because the connection mask didn't always line up with the diagram viewbox since it was always positioned at -pad, -pad.

Details

• have the label mask match the diagram's viewbox which covers outside labels, icons, 3d etc

example of connections being cut off due to a mismatched mask position

with fixed mask position

before

after

[alixander]

oh interesting