Fix intersect lines and polysmooth function

[GiggleLiu]

This PR fixes the following two bugs.

1. The polysmooth has one extra line connecting to it.
2. The intersectlinepoly can not find intersection in some corner case.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (24b2bf9) 76.09% compared to head (bba8e24) 76.10%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #287 +/- ## ========================================== + Coverage 76.09% 76.10% +0.01% ========================================== Files 36 36 Lines 6602 6605 +3 ========================================== + Hits 5024 5027 +3 Misses 1578 1578 ``` | [Files](https://app.codecov.io/gh/JuliaGraphics/Luxor.jl/pull/287?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaGraphics) | Coverage Δ | | |---|---|---| | [src/point.jl](https://app.codecov.io/gh/JuliaGraphics/Luxor.jl/pull/287?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaGraphics#diff-c3JjL3BvaW50Lmps) | `90.16% <100.00%> (ø)` | | | [src/polygons.jl](https://app.codecov.io/gh/JuliaGraphics/Luxor.jl/pull/287?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaGraphics#diff-c3JjL3BvbHlnb25zLmps) | `74.77% <100.00%> (+0.11%)` | :arrow_up: |

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[cormullion]

Thanks!

[cormullion]

In general I think this code/function is probably not very good, but we would have to switch to proper libraries like Clipper2 to make much progress.