Kubuntu, Display Configuration -> Global scale = 125%

The minimize/maximize buttons in Firefox with CSD are smaller than the native ones, see screenshot.

And the close button too, but I never need to hit that one ;-)

Set release status flags based on info from the regressing bug 1812289

This patch on its own doesn't change behavior (as GetWidgetScaleFactor
is still guaranteed to be an integer for now).

Cairo deals with floats just fine. The only scale factor that needs to
be an integer is the style context scale, which is documented to only
affect image resources, so we can just ceil there to have crisp images.

Depends on D168365

KDE uses the GTK text scale factor to implement fractional global
scaling.

While some GTK apps really only use it for text, we interpret it as a
full DPI change (this is configurable via
browser.display.os-zoom-behavior, but that's our default behavior for a
long long time).

We're ignoring the text scale factor right now, but there's no good
reason to it, now that we support fractional scaling. After this patch,
native widgets are consistently scaled with all the other chrome UI
elements. This includes checkbox menuitems, buttons, etc.

The exception here are scrollbars, which are special and still ignore
the text scale factor here:

https://searchfox.org/mozilla-central/rev/8e9b4484408154b80d7ede9e1b035819fda48fd2/widget/ScrollbarDrawing.cpp#28-32

But that's fine because scrollbars are the only widget that ignores full
zoom, and that also matches what Windows does with the text scale
factor, so if we want to change it that's probably worth a separate bug.
(I also have some changes to that code in flight for bug 1812868 so I'd
rather not have gratuituous conflicts).

Depends on D168371

https://hg.mozilla.org/mozilla-central/rev/91afdcafcd9c
https://hg.mozilla.org/mozilla-central/rev/46f5eda5e110