Android) add mechanics to resume application from background

[hsdk123]

Currently, the application will crash if sent to the background and brought back to the foreground.
The following provides a mechanism to continue the application.

[mosra]

If I understand correctly, this is about handling the case where GL context isn't lost and just the window surface is, right? So it's not like existing GL objects become invalid afterwards.

I'll have to think how to expose this in a way that makes sense with other apps. I can imagine that for example on desktop with SDL/GLFW one might want to attach to some focusEvent() / blurEvent() as well, when the window gets active or inactive.

[mosra]

Would this work if you kept the context current and just made it bound to no surface at all?

Suggested change

	/* Pause the context current */
	CORRADE_INTERNAL_ASSERT_OUTPUT(eglMakeCurrent(_display, nullptr, nullptr, nullptr));
	/* Keep the GL context current but without any surface */
	CORRADE_INTERNAL_ASSERT_OUTPUT(eglMakeCurrent(_display, EGL_NO_SURFACE, EGL_NO_SURFACE, _glContext));

Because I'm wondering if it's guaranteed that no app code gets executed between APP_CMD_LOST_FOCUS and APP_CMD_INIT_WINDOW. And if it does, it'd be probably good to have the GL context current.

[hsdk123]

The suggested change, at least from my side, seems to work fine.

[hsdk123]

I'm wondering if it's guaranteed that no app code gets executed between APP_CMD_LOST_FOCUS and APP_CMD_INIT_WINDOW

I'm not sure about there being a guarantee, but it's one of the reasons I added _application_has_valid_surface - such that the user has some info to branch on.

[hsdk123]

@mosra did you find more time to look into this?

[mosra]

Only partially, sorry -- so far I added the focusEvent() and blurEvent() to all apps in 7558bc7, in case you want to adapt this code to use these.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 77.46%. Comparing base (e43eba6) to head (f9d1243).
⚠️ Report is 530 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #660   +/-   ##
=======================================
  Coverage   77.46%   77.46%           
=======================================
  Files         583      583           
  Lines       47828    47828           
=======================================
  Hits        37051    37051           
  Misses      10777    10777           

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