This adds about a frame of latency, and since most games don't change
VI registers during scanout, we can get away with outputting the XFB at
the start of scanout. WWE Crush Hour is the (only currently known)
exception, which has flickering problems when doing it this way.

This adds a path to perform the output at the end of scanout, and gates
it behind an option which defaults to using the latency-reducing
pre-scanout path.

SPDX standardizes how source code conveys its copyright and licensing
information. See https://spdx.github.io/spdx-spec/1-rationale/ . SPDX
tags are adopted in many large projects, including things like the Linux
kernel.

Replace it with a function-local static that is initialized on first
use. This gets rid of a global variable and removes the need for
manual initialization in UICommon.

This commit also replaces the weird find_if that looks for a non-null
unique_ptr with a simple "is vector empty" check considering that
none of the pointers can be null by construction.

Fixes https://bugs.dolphin-emu.org/issues/12245.

I considered making a change to DolphinQt instead of
the core, but then additional effort would've been
required to add the same fix to the Android GUI once
we start using the new config system there.

Fixes https://bugs.dolphin-emu.org/issues/12087

Again, needed for MoltenVK.