ui/sdl: try to instantiate the matching opengl renderer

-display sdl,gl=es didn't actually use OpenGL ES.

Using OpenGL ES allows to use ANGLE, which works generally better than
Windows/OEM OpenGL driver.

(note: users can still bypass the QEMU choice with SDL_RENDER_DRIVER
environment variable)

(note: for some reason, specifying a driver disables batching and
breaks rendering, so enable it explicitly)

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