ui/sdl2: Restore original context after new context creation

SDL API changes GL context to a newly created GL context, which differs
from other GL providers that don't switch context. Change SDL backend to
restore the original GL context. This allows Qemu's virtio-gpu to support
new virglrenderer async-fencing feature for Virgl contexts, otherwise
virglrenderer's vrend creates a fence-sync context on the Qemu's
main-loop thread that erroneously stays in-use by the main-loop after
creation, not allowing vrend's fence-sync thread switch to this new
context that belongs to it.

Reviewed-by: Akihiko Odaki <akihiko.odaki@daynix.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Tested-by: Alex Bennée <alex.bennee@linaro.org>
Acked-by: Pierre-Eric Pelloux-Prayer <pierre-eric.pelloux-prayer@amd.com>
Reviewed-by: Yiwei Zhang <zzyiwei@gmail.com>
Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Message-ID: <20260303151422.977399-6-dmitry.osipenko@collabora.com>
Message-ID: <20260304165043.1437519-8-alex.bennee@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>