Performance: Avoid split.data.frame if there's only one panel

[zeehio]

Splitting a data frame according to a factor with one level will give a list with the data frame.

On layers with 1E7 data points, this split call costs 1.7 seconds out of the 18 seconds it currently takes me to build the plot. I intend to drive those 18 seconds to 6 seconds or less.

I have (maybe too extensively) documented my progress at:

• geom_raster() of a matrix: Performance analysis and improvements #4989

This is the second low hanging fruit after #4990.

Happy to further discuss this 😃

Proposed NEWS entry (not added to avoid trivial conflicts)

* Performance improvement on plots without facets and lots of data
  points (@zeehio, #4991)
``

[teunbrand]

This looks like a sensible step to me in general. Out of scope for this PR however, this makes me wonder whether to apply this trick more broadly. Or even switch to a vctrs::vec_split() based approach, which even for full data.frame splits seem to be about half as expensive as base::split.data.frame().

[zeehio]

Thanks for the code review, @teunbrand .

I applied your suggestion for coercing to factors. I did not go for replacing base::split.data.frame() with vctrs::vec_split() here. It probably makes sense to replace split() with vctrs::vec_split() in multiple places across ggplot2, so I'd rather take care of all those replacements at once, if needed, in a different PR.

[teunbrand]

This looks good to me!