Use higher-resolution icons on HiDPI-friendly backends #4978
Description
On the MacOS X backend, the canvas is rendered at high resolution, but the icons are still low-res:
It would be nice to use higher-res icons on backends that support HiDPI (at least MacOS X, PyQt5, and maybe others)
It looks like the icons are in SVG in lib/matplotlib/mpl-data/images/ so it should be easy to generate new ones.