Divide a circle into four arcs for more precision

[hyrodium]

This PR fixes https://github.com/JuliaGraphics/Luxor.jl/issues/268.

• The number of control points still depends on the radius of the circle. For example, circle(Point(0,0), 500) consists of eight Beziser segments. (Before this PR, it was six segments)
  • This is because Cairo divides the target arcs into Bezier segments based on some atol.
  • I think the best way is adding atol and rtol to Luxor.circle as keyword arguments and approximate the target circle in this package. But I'm not sure that kind of flexibility is desired.
• Some other functions such as pie and arc2r still have the precision issue.
• Checking the precision of the circle would be complicated, so I have not added tests for this change.

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.01% :tada:

Comparison is base (5e81f15) 73.16% compared to head (8584fba) 73.17%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #269 +/- ## ========================================== + Coverage 73.16% 73.17% +0.01% ========================================== Files 34 34 Lines 6753 6756 +3 ========================================== + Hits 4941 4944 +3 Misses 1812 1812 ``` | [Files Changed](https://app.codecov.io/gh/JuliaGraphics/Luxor.jl/pull/269?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaGraphics) | Coverage Δ | | |---|---|---| | [src/curves.jl](https://app.codecov.io/gh/JuliaGraphics/Luxor.jl/pull/269?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaGraphics#diff-c3JjL2N1cnZlcy5qbA==) | `96.24% <100.00%> (+0.02%)` | :arrow_up: |

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[hyrodium]

Thank you for the merge!