Rendering artifact at the start of a `geomLine()` stroke when exporting with `ggsave()`

Example:

```kotlin
// The line thickness has been increased slightly to make it easier to see
val p = letsPlot() + geomLine(size = 5) { x = listOf(-1, -1, 1, 1); y = listOf(-1, 1, 1, -1) }
ggsave(p, "plot.png")
```

Output:

<img width="600" height="400" alt="Example" src="https://github.com/user-attachments/assets/738df828-a6c6-4150-be4a-f19cf9f7a14b" />