Fix math error in clipping code

[khenriks]

ymax should be the ceil of the argument, not the floor.

[codecov]

Codecov Report

Merging #199 (b15377b) into master (7225725) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #199   +/-   ##
=======================================
  Coverage   68.44%   68.44%           
=======================================
  Files          27       27           
  Lines        6050     6050           
  Branches     1094     1094           
=======================================
  Hits         4141     4141           
  Misses       1645     1645           
  Partials      264      264           

Flag	Coverage Δ
unittest	68.44% <100.00%> (ø)

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Impacted Files	Coverage Δ
gerber/render/cairo_backend.py	35.02% <100.00%> (ø)

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[rusefillc]

#228