showtext text sizes are wrong when png(type='cairo')

[cpsievert]

(Related to yixuan/showtext#33)

Minimal reprex (must be run on a machine without aqua capabilities and viewed on a high-resolution display):

library(shiny)
library(showtext)
font_add_google("Gochi Hand", "gochi")
showtext_auto()

# showtext's dpi option currently doesn't work with CairoPNG (see issue above for more details)
opts <- options(shiny.usecairo = FALSE)
onStop(function() { options(ops) })

shinyApp(
  ui = fluidPage(
    numericInput("n", "n", 1),
    plotOutput("plot")
  ),
  server = function(input, output) {
    output$plot <- renderPlot({
      plot(head(cars, input$n), family = "gochi")
      title("Hello world")
    })
  }
)

Notice how the text is much smaller than it should be because showtext is unaware of the device resolution. Shiny needs to set showtext::showtext_opts(dpi = ...) with the resolution it passes to the graphics device for this to work properly.

[cpsievert]

Fixed in #2740