Make plot update smarter using magic null dev size

[renkun-ken]

What problem did you solve?

Closes #271

This PR makes plot update smarter using a magic null dev size so that we can check if the current device is the NULL device we create as the default device to output graphics. I don't see if there's any better way to help us identify the device we create since user has very limited capability to set metadata and limited access to the information of a device.

Now plot file is open only when graphics functions are called with the NULL pdf device we create. So knitr::knit() no longer triggers opening existing plot file, and user created device such png or other pdf will not set plot_updated = TRUE so VSCode won't copy the graphics from the current device.

(If you have)Screenshot

(If you do not have screenshot) How can I check this pull request?

• Basic plot

plot(rnorm(100))
abline(h = 0, col = "red")
# close the plot file
png("test.png") # should not open plot file
plot(rnorm(100)) # should not open plot file
dev.off() # should not open plot file
abline(v = 0, col = "blue") # should open the plot file

• knitr::knit()

1. Create a Rmd document (e.g. doc1.Rmd) with the following content:

# hello

Hello, world!

```{r}
plot(rnorm(100))
```

2. Call knitr::knit("doc1.Rmd")
3. When the function call completes, no plot file appears.
4. Call plot(rnorm(10)) and the plot file is open. Close this file.
5. Call knitr::knit("doc1.Rmd") again and the previous plot file is not open.

[andycraig]

@renkun-ken I tested this on Windows 10 and it works. If you are happy with it, please merge.

[renkun-ken]

Thanks for testing!