Restore previously opened device

[hadley]

Fixes #138

[gaborcsardi]

Do we want to handle this case?

f <- function() { 
  withr::local_pdf(tempfile())
  print(dev.cur())
  png("4.png")
  dev.cur() 
}
png("2.png")
png("3.png")

dev.cur()
#> quartz_off_screen
#>                 3

f()
#> pdf
#>   4
#> quartz_off_screen
#>                 5

dev.cur()
#> quartz_off_screen
#>                3

I.e. if the current device changes between the creation and the destruction of the temporary device, should we keep that new device, instead of restoring the one that was active before the temporary device. I think that would be more natural, and to do it we would need to check if we are closing the current device.

(And of course, if somebody closes the temporary device and creates a new one, then our exit handler will close that one, but since the device id is a simple number which is reused, we cannot do much about this.)

[lionel-]

I.e. if the current device changes between the creation and the destruction of the temporary device, should we keep that new device, instead of restoring the one that was active before the temporary device. I think that would be more natural, and to do it we would need to check if we are closing the current device.

There is probably no ideal way here, so I think we should favour the simplest / most obvious behaviour. I think I like the guarantee that you can go back to working on the device that was active on entry once you exit a local_ / with_ context. It seems least surprising to me. Does that make sense?