This code is to be run with R 4.2.0 to produce images that reflect
the old behaviour of Cairo compositing operators.

<!--begin.rcode id="init", echo=FALSE, message=FALSE, results="hide"
library(knitr)
opts_chunk$set(comment=" ", tidy=FALSE, 
               dev="png", ## dev="svg",
               dpi=96)
end.rcode-->
    
<!--begin.rcode
library(grid)
end.rcode-->

<!--begin.rcode
src <- rectGrob(x=2/3, y=1/3, width=.5, height=.5,
                gp=gpar(col=NA, fill=rgb(0, 0, .9)))
dst <- rectGrob(x=1/3, y=2/3, width=.5, height=.5,
                gp=gpar(col=NA, fill=rgb(.7, 0, 0)))
end.rcode-->

<!--begin.rcode
group <- function(src, op, dst) {
    pushViewport(viewport(width=.95, height=.95))
    grid.rect()
    grid.group(src, op, dst)
    grid.text(op, x=.1, y=.1, just=c("left", "bottom"),
              gp=gpar(cex=.7, fontfamily="HersheySans"))
    popViewport()
}
end.rcode-->

<!--begin.rcode oldCairo, fig.width=4, fig.height=2
pushViewport(viewport(x=0, width=.5, just="left"))
group(src, "clear", dst)
popViewport()
pushViewport(viewport(x=.5, width=.5, just="left"))
group(src, "source", dst)
popViewport()
end.rcode-->