Framebuffer access control

[cepetr]

This small PR adds precise access control for the framebuffer (and resolves #4207).

This change is primarily aimed at devices using two framebuffers (T3T1) but also affects devices using a single framebuffer (T2B1, T3B1).

Before this change, the unprivileged app had access to both framebuffers at any time. Now, an unprivileged application has access only to the framebuffer acquired by display_get_frame_buffer() until it releases it by calling the display_refresh() function.

[github-actions]

core UI changes	device test	click test	persistence test
T2T1 Model T	test(screens) main(screens)	test(screens) main(screens)	test(screens) main(screens)
T3B1 Safe 3	test(screens) main(screens)	test(screens) main(screens)	2724
T3T1 Safe 5	test(screens) main(screens)	test(screens) main(screens)	test(screens) main(screens)
All	main(screens)

[Hannsek]

What is the diff here? Only the 4 pixels?

[cepetr]

What is the diff here? Only the 4 pixels?

It seems like a recurring issue with the loader. @TychoVrahe, you fixed this a few weeks ago, didn’t you?

[TychoVrahe]

I created this PR to fix that, it hasn't been merged as another solution, via GFL, is preffered.

See also slack discussion: https://satoshilabs.slack.com/archives/C054XGU77QV/p1724926329014489

Lets ignore this here.