A penny is a thin layer of copper around a bunch of zinc. The copper here is some glue between libGLX/libEGL and zink to translate the GL winsys operations into Vulkan operations. This works as an intermediate fallback between dri2/3 for native hardware acceleration and drisw for software fallback; if you don't have a native GL driver, but you do have Vulkan, you can do this instead.
This will hopefully address both #3608 (closed) and #3178 (closed), and pave the way for the glorious future in which we just use zink as the GL driver on new platforms. It probably also has some impact on the zink-on-moltenvk work since both things will be touching the winsys bits of zink.
This gets as far as glxinfo on NVIDIA's driver but does not actually have any drawable operations wired up yet, nor have I started really working on EGL yet either.