Implement OS.get_screen_dpi() on iOS

[HEAVYPOLY]

Godot version:
3.2 stable
OS/device including version:
iOS
Issue description:
OS.get_screen_dpi(OS.get_current_screen()) yields wrong value on iOS
using iPad Pro first generation, dpi is returned as 72, when in reality it is 264
Same code works correctly on MacOSX, Windows, and Android devices
Steps to reproduce:

Minimal reproduction project:

[Calinou]

get_screen_dpi() isn't implemented on iOS, which means the fallback value of 72 is returned.

This is already documented here:

godot/doc/classes/OS.xml

Line 331 in 213a855

[b]Note:[/b] This method is implemented on Android, Linux, macOS and Windows. Returns [code]72[/code] on unsupported platforms.

[HEAVYPOLY]

Thanks for the clarification, excited for this enhancement!

[ViktorEvil]

Need this for a pixel art game I’m making for iOS, my scaling script to get pixel perfect art has different results on devices that use @3 resolution. My workaround involves using OS.get_model_name and setting the scaling depending on a list of all iOS devices but this isn’t a very elegant solution

[naithar]

Was implemented for 3.2 in #42582. Should be available in 3.2.4 RCs.
@HEAVYPOLY can you please check if it's working?

[HEAVYPOLY]

@naithar getting correct 264 dpi on 1st gen iPad Pro. Thank you!

[Calinou]

This was implemented in 3.3 and later per the above comment, closing.