Upcoming scales 1.4.0 release

[teunbrand]

The upcoming release of scales (r-lib/scales#474) prompts a few tasks on ggplot2's side.

• Prepare for future incorporation of breaks_exp() into transform_exp() (Use snapshot expectations for transformation breaks #6171)
• Remove shims like the following

  ggplot2/R/utilities.R

  Lines 792 to 808 in b29b831

  	# Shim for scales/#424
  	col_mix <- function(a, b, amount = 0.5) {
  	input <- vec_recycle_common(a = a, b = b, amount = amount)
  	a <- grDevices::col2rgb(input$a, TRUE)
  	b <- grDevices::col2rgb(input$b, TRUE)
  	new <- (a * (1 - input$amount) + b * input$amount)
  	grDevices::rgb(
  	new["red", ], new["green", ], new["blue", ],
  	alpha = new["alpha", ], maxColorValue = 255
  	)
  	}
  	on_load({
  	if ("col_mix" %in% getNamespaceExports("scales")) {
  	col_mix <- scales::col_mix
  	}
  	})

• Tweak col_mix() calls to continue to produce the same hex codes, because the shim has a grDevices::rgb() backend while scales::col_mix() is based on {farver}, producing tiny differences.
• Rework default colour/fill scales to accept new palette specifications (More palette flexibility #6112)
• Ensure that default palettes can be retrieved from the theme (Scale palettes from theme #5946)