Provide macOS style fractional scaling

[mixedCase]

Some users prefer the result of fractional scaling the macOS way, where rather than letting surfaces control their fractional rendering, they're requested to render at a higher native resolution and downscale using Lanczos, to the tune of:

$$\text{Render Scale} = \left\lceil \frac{\text{UI Scale} \times \text{Native PPI}}{\text{Reference 1x PPI}} \right\rceil$$

or alternatively: int render_scale = std::ceil(ui_scale * native_ppi / reference_1x_ppi);

Meaning, a 27" 4k monitor configured for 1.5x would render scale at 3x, and then downscaled /2 to fit in its native 4k neatly while giving it the physical size you'd expect of a 27" 1440p monitor at 1x.

Whether the best option would be to allow this as a window rule and/or monitor rule beats me, macOS only does it at the display level, but then again macOS does not run Wayland and thus there's no "each toolkit supports fractional scaling their own way".

Feel free to poke holes at the formula and the idea. Full disclosure: I'm not even the target user for this since I shop my monitors with integer scaling in mind, but I've talked with enough people both IRL and online complaining about this about for desktop Linux in general, and today a straw broke the camel's back.

[vaxerski]

ignoring how bad this method is, that's the default on wayland unless an app supports proper fractional scaling.

[mixedCase]

I think the difference is that (IIUC, correct me if I'm wrong) that behavior scales to nearest integer and uses bilinear filtering. This would scale rendering to 3x in a 1.5x situation for the mentioned 27" 4k monitor, instead of 2x and use Lanczos resampling which is what gives them the exact behavior they're comfortable with.

And of course, this kind of behavior would be configurable to:

a) Make sure it applies to everything the user explicitly wants (e.g. all surfaces in a specific monitor, and/or according to window rules), purposefully not requesting each app to use their fractional scaling implementation.

b) Not affect anyone who wants fractional scaling done by each application/toolkit, by virtue of being a modifier to the scale column or require something else to be set.

To continue playing Devil's advocate, I can understand not wanting to deal with some applications that do fractional rendering in a completely retarded manner, and asking the compositor do the lifting from stupid land back to something usable. Hell, it'd be great if something like this could work for XWayland HiDPI situations and multi-monitor support, which I also solved by giving up on HiDPI+multi-monitor completely, but it's not the most realistic ask for most people who already have such set-ups.