feat: iOS Picture in Picture support

[23doors]

PIP with texture API is only available since iOS 15+ so this is a must.

To enable PIP app needs to have audio background mode enabled in Info.plist:

	<key>UIBackgroundModes</key>
	<array>
		<string>audio</string>
	</array>

and that's pretty much it. New APIs:

controller.isPictureInPictureAvailable() -> only returns true on iOS15+ platform
controller.enablePictureInPicture()
controller.disablePictureInPicture()
controller.enableAutoPictureInPicture() -> when app goes into background, currently playing video will automatically go into PiP
controller.disableAutoPictureInPicture()
controller.enterPictureInPicture() -> requires PiP to be enabled first (or auto PIP)

[birros]

I just tested it, awesome!

However, I have encountered some bugs:

1. sometimes the PIP displays a black screen, especially when I switch to network videos, but I have trouble reproducing
2. using software rendering when the app is unfocused causes colorimetry problems with HDR videos, but this problem also exists with non-PIP software rendering. @23doors are you sure there is no way to keep hardware rendering when the app is unfocused?

---

Finally, there is still some work to do before we can merge:

1. @alexmercerind needs to confirm that he's ok with adding this kind of API to the VideoController, maybe review the syntax
2. certainly review the VideoOutput.swift code, probably by moving it to a dedicated file…

[23doors]

Regarding 1. Does it fix itself or stays black? I can try to fix it if it is reproducable but as you know, I have near zero experience with MacOS/iOS and swift in general.

Regarding 2. Yes sadly this seems to be the case. Any kind of opengl calls when app is in background cause a bunch of warnings about app not having access to GPU. This is also consistent with what I found in docs about iOS background mode.

https://developer.apple.com/documentation/metal/gpu_devices_and_work_submission/preparing_your_metal_app_to_run_in_the_background

iOS and tvOS restrict a background app’s access to the GPU, to guarantee foreground app performance.

Regarding review.
Feel free to suggest whatever you feel is necessary. This is pretty much the first time I touched swift. Separated VideoOutputPIP.swift for now

[birros]

So, after 2 weeks of considerations, I'm not OK to merge this PR until a solution is found for HEVC videos rendering in PIP:

• by finding a way to enable the GPU when the app is in background
• or by fixing colors in software rendering

If we merge with this "bug", people will think this feature is ready, and will lead to countless issues about it…

@23doors sorry, that doesn't stop you from keeping your branch up to date, until we find a solution, or someone help us…

@alexmercerind I set this PR as draft with the help-wanted label if you agree.

[alexmercerind]

Hi! I apologize for the delay.

I'm still thinking over the code style. Ideally, we'd land this after the first-stable 1.0.0 release, once it's well-tested & (hopefully) implemented for other platforms too.

[kkkkkkkkkkkkkkeee]

Excuse me, everyone. Is this still going on

[AlvaroVasconcelos]

I really need this functionality, is there anything new?