[fence_manager, ui] antiflicker fix + toggle integration (#4004)

tldr: we (me and maran) found the missing piece to make delay_fence and should_flush coop and cover all remaining flicker issues tested so far.
the lil change is in src/video_core/fence_manager.h
the rest is toggle stuff.

END

(credits rolls up)

post credits scene for the toggle drama:

i've been forced to use custom builds for around 1 year now, coz gpu mode above fast was pulling performance down yet not covering flicker entirely in most games tested. and we know that it's a mess to rely on gpu mode as it brings along a bunch of undesired stuff.
i need this toggle. and i've seem far less relevant before, so hey, gimme this one so i can call it mine :3

Reviewed-on: https://git.eden-emu.dev/eden-emu/eden/pulls/4004
Reviewed-by: MaranBr <maranbr@eden-emu.dev>
Reviewed-by: Lizzie <lizzie@eden-emu.dev>
Reviewed-by: CamilleLaVey <camillelavey99@gmail.com>

src/common/settings.h

@@ -545,6 +545,13 @@ struct Values {
                                                         Specialization::Default,
                                                         true,
                                                         true};
+    SwitchableSetting<bool> antiflicker{linkage,
+                                        false,
+                                        "antiflicker",
+                                        Category::RendererHacks,
+                                        Specialization::Default,
+                                        true,
+                                        true};
     SwitchableSetting<bool> async_presentation{linkage,
 #ifdef ANDROID
                                                false,