'grid' graphics updates: documentation for #7, #35 and #49.

[aphalo]

Implementing the enhancements in Issues #49, #35, #7, and possibly additional enhancements are made possible or simpler by recent updates to 'grid' graphics in R 4.1.0, 4.2.0 and 4.3.0, which are documented in the reports listed below.

For shape clipping and combining see:
https://www.stat.auckland.ac.nz/~paul/Reports/Geometry/coords/coords.html
For luminance masks see:
https://www.stat.auckland.ac.nz/~paul/Reports/GraphicsEngine/masks/masks.html
https://www.stat.auckland.ac.nz/~paul/Reports/GraphicsEngine/groups/groups.html
For gradient fills:
https://www.stat.auckland.ac.nz/~paul/Talks/useR2021/index.html
For clipping:
https://www.stat.auckland.ac.nz/~paul/Reports/GraphicsEngine/r-clipping/r-clipping.html
for 'gridGeometry' see:
https://www.stat.auckland.ac.nz/~paul/Reports/Geometry/gridgeometry/gridgeometry.html
https://www.stat.auckland.ac.nz/~paul/Reports/Geometry/offset/offset.html
for built-in compositing and paths see:
https://www.stat.auckland.ac.nz/~paul/Reports/GraphicsEngine/compositing/compositing.html
https://www.stat.auckland.ac.nz/~paul/Reports/GraphicsEngine/paths/paths.html