Round join rendered incorrectly at <3px wide strokes in 0.92

[Alexandre Prokoudine]

1. Draw a closed path.
2. Change its stroke width to 1px or 2px, use round join.
3. Zoom in and look at the node where you closed the path: it won't render correctly.
4. Change stroke width to 3px or larger. Now it works as expected.

[su_v (suv-lp)]

Likely a duplicate of
* Bug #1409520 “bad rendering of rounded corners for small objects”
  https://bugs.launchpad.net/inkscape/+bug/1409520
(as far as is known an issue with more recent versions of cairo)

[su_v (suv-lp)]

Tests with various archived builds (on OS X 10.7.5):
- not reproduced with Inkscape 0.48.5 r10040,
- not reproduced with lp:inkscape r14653, cairo 1.12.2,
- reproduced with lp:inkscape r14653, cairo 1.12.14,
- reproduced with lp:inkscape r14653, cairo 1.14.6,
- reproduced with Inkscape 0.91 r13725, cairo 1.14.0,
- reproduced with Inkscape 0.92 r15299, cairo 1.14.6,
- reproduced with lp:inkscape/0.92.x r15346, cairo 1.15.5;
matches the findings in bug #1409520 and bug #1505501 (earlier duplicate).