Release v0.13.0

What's Changed

• ci: use Go-1.19 in codecov by @sbinet in #755
• ci: explicitly use ubuntu-20.04 by @kortschak in #757
• Adds plotutil example image width/height unit by @zyluo in #756
• mod: update golang.org/x/image version by @kortschak in #759
• all: bump to Go-1.20, drop Go-1.18 by @sbinet in #760
• plotter: implement GlyphBoxer for plotter.Line by @sbinet in #762
• vggio more tests by @sbinet in #765
• all: update to latest Gio API by @sbinet in #764
• all: update dependencies for Gonum-v0.13.0 by @sbinet in #766

New Contributors

• @zyluo made their first contribution in #756

Full Changelog: v0.12.0...v0.13.0