render: fix connection stroke clipping

[gavin-ts]

Summary

Fixes rendered connections that were clipped by the mask at the top and left edges of the graph.

Details

• fixes render: bounding box needs to consider edge stroke width #488
• the connection mask correctly had the same dimensions as the viewbox but we positioned it at 0,0 instead of at -pad,-pad to match the placement of the viewbox
• removed unused code that previously accounted for connection stroke width when we masked each connection individually
• test changes are just from the updated mask position

tala self loop in top left corner

before

after

[ejulio-ts]

for a moment I thought it was an issue computing the edge bounding box and not with rendering 😅