Fill small donut shape

[norihiro]

When using bold outline, donuts shape whose size is smaller than the outline width is not filled.

This PR is under development.

Below is prior and current drawing results with white text and black outline with several outline width.

Prior	Current
Donut shapes smaller than the outline width is not drawn.	Donut shapes is always filled. Still circles whose diameter is smaller than the outline width is not properly outlined.

Fix #14.

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