Use stb_image_write to save PNG screenshots

[oznogon]

Add the stb_image_write header-only library to write a screenshot to a PNG file on a background thread. Add Window::saveScreenshotToFile(w,h) function and screenshot_key keybinding.

This allows for a few advantages over OS screenshot capabilities:

• Works when rendering via KMSDRM
• No OS chrome, drop shadows, etc. in screenshots; this saves only what's in the framebuffer
• Should work per-window, but no multi-monitor setup to test behavior on

If the additional vendored lib is undesirable, this could be implemented without stb_image_write to save uncompressed TGA or BMP screenshots, but the sizes of them would be dramatically larger (about 9MB on a display of 2.7k pixels vs. ~700-900kb PNG).

stb_image_write is monkey patched to avoid CI failing on -Wextra warnings from nothings/stb#1099.

nothings comment from 2021 on issue 1099:

Our general rule has been that it's not generally worth fixing stuff that only shows up with -Wextra, but that's because fixes aren't always straightforward and can even conflict. In this case, since there's a straightforward way to fix it, and given that it seems unlikely clang is going to change it's mind about warning for this, I would accept a PR fixing it.

Upstream PR nothings/stb#1311 (2022, still open) implements this change.

[daid]

I'm wondering if I'm fine with this or rather have it as part of the Image class as in SP2: https://github.com/daid/SeriousProton2/blob/master/src/graphics/image.cpp#L132