Submit knitr PR

[hadley]

• To depend on evaluate 1.0.0, when released
• Use trim_intermediate_plots()
• Add issue to remove evaluate's .onLoad() code in a year or two.

[hadley]

First attempt at yihui/knitr#2363

[hadley]

Need to consider rgl's is_low_change() method before we can make progress on this. There don't appear to be other methods: https://github.com/search?q=org%3Acran%20is_low_change&type=code