ui/sdl: get the GL context from the window

ui/sdl: get the GL context from the window

There is no guarantee to have a current GL context here. The current
code seems to rely on the renderer using a GL backend, and to set a
current GL context. But this is not always the case, for example if the
renderer backend is DirectX.

This change is enough to fix using virgl with sdl2 on win32, on my setup.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>