Changes to allow rgl (and other?) graphics systems to work like base graphics

[dmurdoch]

This patch is related to my comments on #1881 and my issue #1853, to allow rgl to work more seamlessly with knitr. The idea is that, like base graphics, rgl has "high level" and "low level" commands to change the current display, and users typically only want to see output when a new high level command is run.

To do this, I needed to export two internal functions from knitr: wrap() and is_low_change(). I also needed to change is_low_change() to a generic function, so the test for low versus high could take place in rgl, and I introduced a new class name, "knit_other_plot" so that knitr could tell that my rgl graphics objects are actually plots that need handling. The access to wrap() allows rgl graphics to efficiently share data: if part of a plot has previously appeared, it won't be written to the HTML output.

This PR shouldn't have bad effects on anyone as far as I know. The current development version of rgl on R-forge doesn't take advantage of the changes; it uses a complicated (and fragile, I worry) set of hooks to replace a bunch of code in knitr. You can see the code that does work with this PR in https://github.com/dmurdoch/rgl branch rglpatch.

[dmurdoch]

Travis seems confused: some checks have been sitting there for a few days, stuck at "Queued -- Build Created".

[CLAassistant]

All committers have signed the CLA.

[dmurdoch]

The main branch of rgl on github.com/rforge/rgl now supports this PR, as well as the version of knitr on CRAN. I'd like to hold off a CRAN release of rgl until this PR is accepted (or not), in case details change.

[dmurdoch]

Similar changes to these were merged into pkgdown in r-lib/pkgdown#1548 and r-lib/downlit#78 . Some differences:

• In this PR, I exported wrap and is_low_change, but in the pkgdown changes the corresponding functions (downlit:::replay_html and downlit:::is_low_change) weren't exported.

• The new class name "knit_other_plot" was called "otherRecordedplot".

If you'd like one or both of those changes in this PR, I'd be happy to make them.

[yihui]

Looks good to me overall. I'll make some minor and doc changes before merging. Thanks!

[dmurdoch]

Great!