glGenVertexArrays requires opengl >= 3.0

and since...

commit c831e485
Date:   Wed Dec 9 21:39:27 2015 +0000

    slideshow: Fix what was remaining to be able to use a core context

we always use that in all slide transitions, so slideshow opengl transitions
presumably thus all have a min opengl version of 3.0

Change-Id: Ifb21201fc94c4a911c8d4127d1df1efb3e34a2d8
Reviewed-on: https://gerrit.libreoffice.org/59298
Tested-by: Jenkins
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>