Divide a circle into four arcs for more precision

[hyrodium]

This PR fixes #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]

Codecov Report

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

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

Additional details and impacted files

@@            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	Coverage Δ
src/curves.jl	96.24% <100.00%> (+0.02%)	⬆️

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[hyrodium]

Thank you for the merge!