fix: deviceScaleFactor = 0 disables the metric override

[gadenbuie]

Fixes rstudio/shinytest2#404

In rstudio/shinytest2#350, we went from using deviceScaleFactor = 1 before screenshots to using deviceScaleFactor = 0 in the Emulation.setDeviceMetricsOverride command. As pointed out there, the CDP docs explain that this is equivalent to disabling deivce scaling.

The issue rstudio/shinytest2#350 resolved was a problem that was flagged for us on CRAN that was difficult to reproduce. IIRC, we eventually realized that we were getting different screenshot sizes on Mac when the Chrome browser window was being displayed on a Retina display vs a monitor (and by association, the failing manual CRAN tests were happening on a laptop display).

That said, playwright defaults to deviceScaleFactor = 1: https://github.com/microsoft/playwright/blob/3b9515e7efa07230684bef9704c601f509f8ad72/packages/playwright-core/src/server/chromium/crPage.ts#L964

Anyway, for consistently sized screenshots, we may end up in a situation where we need to query window.devicePixelRatio to find the current value (because deviceScaleFactor was set to 0).

In retrospect, the real issue behind rstudio/shinytest2#350 was that we were calling Emulation.setDeviceMetricsOverride and setting deviceScaleFactor = 1, but internally the screenshot method was still using pixel_ratio = 2 because we weren't tracking changes to pixel_ratio. So using deviceScaleFactor = 0 in shinytest2 was equivalent to not changing the pixel ratio.

We could consider not tracking pixel_ratio at all, but our current approach does save us a round trip call or two to the browser whenever we change the viewport size.

[gadenbuie]

Just a note that deviceScaleFactor basically overrides window.devicePixelRatio

pkgload::load_all()
#> ℹ Loading chromote

b <- ChromoteSession$new()
b$Runtime$evaluate("window.devicePixelRatio")$result$value
#> [1] 1

b$set_viewport_size(800, 800, zoom = 2)
b$Runtime$evaluate("window.devicePixelRatio")$result$value
#> [1] 2

b$set_viewport_size(800, 800, zoom = 1.25)
b$Runtime$evaluate("window.devicePixelRatio")$result$value
#> [1] 1.25

where deviceScaleFactor = 0 is equivalent to removing any overrides, in which case we set private$pixel_ratio = NULL as a signal that we need to check in with the browser to know the current value.

b$set_viewport_size(800, 800, zoom = 0)
b$Runtime$evaluate("window.devicePixelRatio")$result$value
#> [1] 1

[gadenbuie]

Merging so we get overnight tests in shinycoreci; the failing test is flaky and I'll follow up separately to address that.