Recommend the platform's default video backend in the Backend tooltip
instead of always recommending OpenGL.

Rename GetDefaultBackendName to GetDefaultBackendConfigName to
distinguish it from the display name.

Remove the PopulateBackendInfoFromUI function, which had a single caller
(GraphicsWindow::OnBackendChanged) and checked that the core wasn't
running or starting before calling PopulateBackendInfo.

Move the core state check into PopulateBackendInfo and have
OnBackendChanged call that instead. This guarantees the check is
performed by all callers of PopulateBackendInfo, preventing
potential reintroduction of the crash fixed in 3d4ae63f if another call
to PopulateBackendInfo is added.

As of the previous commit the only other caller of PopulateBackendInfo
is Core::Init shortly before s_state is set to Starting, so it will
always pass the check and so maintain its current behavior.

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.