core: improve DPI awareness & add docs

[slimsag]

Today mach-core provides:

1. Framebuffer size in pixels (arbitrary units)
2. Window size in pixels (arbitrary units)
3. A guarantee that framebuffer size is always >= window size.
   • Example A: on a non-retina/HDPI display, the framebuffer size and window size may be the same.
   • Example B: on a retina/HDPI display, the framebuffer size may be twice the window size.

The following issues exist:

• [ ] https://machengine.org/core should have documentation about how mach-core handles resolutions, DPI and Retina displays, etc.
• [ ] on MacOS, today we always choose to have the framebuffer size match the actual native resolution (e.g. 2x the window size), see [0] [1], this should be made configurable.
• [ ] on Windows, we currently do not have a way to opt-in to e.g. "Per-Monitor (V2) DPI Awareness"
• [ ] on Linux, it is unclear how mach-core/GLFW behaves currently when using e.g. fractional scaling in gnome or similar.

[pdoane]

DXGI_SCALING_NONE might want to be tracked elsewhere. It's a nice to have during resize events to give better stability.

[slimsag]

Done! https://github.com/hexops/mach/issues/1105