p5: better handling of closed paths

Detect shapes that are expected to be closed paths and properly close
the Gio path.
This fixes not quite closed paths because of float32 rounding errors.

Fixes #63.

Signed-off-by: Sebastien Binet <binet@cern.ch>

ci: bump to Go-1.17, bump golangci-lint@v1.42

p5: hack for TestCurve

On GitHub Actions CI, Gio (actually, libGL) falls back on using s/w renderer.
The output doesn't look much like the GPU-based one for TestCurve.
Workaround this by increasing the img-diff delta.

example/anim-image: use embed directive

example/anim-image: use net/http to fetch Gopher image

example/emfield: first import

example/anim-image: first import

p5: bump Gio

Signed-off-by: Sebastien Binet <binet@cern.ch>

p5: add Matrix context

p5: enable gproc testing