Use a software renderer for kittest

[abey79]

Kittest-based UI snapshot test currently use whatever hardware backend wgpu picks, which has multiple drawbacks:

• each backend may produce a slightly different output, making comparison more difficult
• a hardware backed is required, which rules out windows and linux GHA runners

Using a software rendering backend would solve both of these issues.

[emilk]

each backend may produce a slightly different output, making comparison more difficult

According to @Wumpf this is likely due to differences in filtering. If we use a manual four-tap bilinear filter in the egui pixel shader we should get a lot closer results, hopefully within the error margins.

[Wumpf]

Using a software rendering backend would solve both of these issues.

Unfortunately not true since software renderer among each other also have different results. E.g. llvmpipe won't give use the same as microsoft's WARP

[emilk]

So we're back to the manual-texture-filtering idea then?

[Wumpf]

That would surely alleviate the issue(s) significantly (very much in favor of trying that for the egui shader!), but we'll for sure never catch all differences anyways 🤷 - we can't realistically catch all sources of rendering discrepancies everywhere.
I don't think there's a remedy for tweaking error thresholds