'maskGrob()'

[trevorld]

• Support masking one grob via another grob

  • Support both "alpha" and "luminance" masking
  • Option to use either new R 4.1 "alpha" or R 4.2 "luminance" masking features or perform a manual rasterGrob() approximation (which should work on a wider variety of R versions and/or graphic devices)

• Main use case would be support https://github.com/coolbutuseless/ggpattern/issues/69 in a way that would be compatible with a wider variety of R versions and/or graphic devices

  • However unclear whether using masks instead of clipping paths would actually solve any of our "problems"

• References on R 4.1/4.2 masks as well as R 4.1 features in general:

  • https://www.stat.auckland.ac.nz/~paul/Reports/GraphicsEngine/masks/masks.html
  • https://www.stat.auckland.ac.nz/~paul/Reports/GraphicsEngine/definitions/definitions.html

[trevorld]

Just supporting alphaMaskGrob() for now (which is all we currently need).