fix: Wrong flipped start arrowhead X offset

[wodeni]

Description

Fix #1220

The renderer doesn't compute the X-offset of line start with a flipped arrowhead correctly. This PR fixes this issue.

Implementation strategy and design decisions

Include a high-level summary of the implementation strategy and list important design decisions made, if any.

Examples with steps to reproduce them

https://fix-arrow-flip.penrose-72l.pages.dev/try/?gist=0d23bb22d18cfebcda7d9febd2513089

Checklist

• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new ESLint warnings
• I have reviewed any generated registry diagram changes

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± Registry diff

📊 Performance

Key

Note that each bar component rounds up to the nearest 100ms, so each full bar is an overestimate by up to 400ms.

     0s   1s   2s   3s   4s   5s   6s   7s   8s   9s
     |    |    |    |    |    |    |    |    |    |
name ▝▀▀▀▀▀▀▀▀▀▀▀▚▄▄▄▄▄▄▄▄▄▞▀▀▀▀▀▀▀▀▀▀▀▀▚▄▄▄▄▄▄▄▄▄▖
      compilation labelling optimization rendering

Data

                                        0s   1s   2s   3s   4s   5s   6s   7s   8s
                                        |    |    |    |    |    |    |    |    |
3d-projection-fake-3d-linear-algebra    ▝▞▖
allShapes-allShapes                     ▝▚▚▄▖
arrowheads-arrowheads                   ▝▞▖
caffeine-structural-formula             ▝▀▀▀▚▀▖
center-shrink-circle-animation          ▝▞▖
circle-example-euclidean                ▝▀▚▀▀▀▚
collinear-euclidean                     ▝▀▞▖
congruent-triangles-euclidean           ▝▀▀▚▚
continuousmap-continuousmap             ▝▚▚
full-moon-full-moon                     ▝▀▀▀▞▀▀▖
glutamine-molecules-basic               ▝▀▚▚
half-adder-distinctive-shape            ▝▀▞▚
hypergraph-hypergraph                   ▝▀▀▀▞▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▖
incenter-triangle-euclidean             ▝▀▀▞▖
lagrange-bases-lagrange-bases           ▝▚▚
midsegment-triangles-euclidean          ▝▀▞▖
mobius-mobius                           ▝▞▖
non-convex-non-convex                   ▝▀▞▖
one-water-molecule-atoms-and-bonds      ▝▚▚
parallel-lines-euclidean                ▝▀▞▖
persistent-homology-persistent-homology ▝▀▀▞▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▚▖
points-around-line-shape-distance       ▝▀▚▚
points-around-polyline-shape-distance   ▝▀▀▞▖
points-around-star-shape-distance       ▝▀▀▀▀▞▖
siggraph-teaser-euclidean-teaser        ▝▀▚▚
small-graph-disjoint-rect-line-horiz    ▝▀▀▞▖
small-graph-disjoint-rects              ▝▀▞▖
small-graph-disjoint-rects-large-canvas ▝▚▚
small-graph-disjoint-rects-small-canvas ▝▚▚
tree-tree                               ▝▀▞▖
tree-venn                               ▝▀▀▞▖
tree-venn-3d                            ▝▀▞▄▄
two-vectors-perp-vectors-dashed         ▝▚▚
vector-wedge-exterior-algebra           ▝▚▚
wet-floor-atoms-and-bonds               ▝▀▞▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▀▖
word-cloud-example-word-cloud           ▝▀▞▖
wos-laplace-estimator-walk-on-spheres   ▝▀▀▞▖
wos-nested-estimator-walk-on-spheres    ▝▀▀▚▀▀▀▀▀▀▀▀▀▖
wos-offcenter-estimator-walk-on-spheres ▝▀▀▞▀▀▀▚
wos-poisson-estimator-walk-on-spheres   ▝▀▚▀▚