Use the standard functions in GLES3 builds in VAOs

There is no guarantee that the OES extension is present and the standard
functions are not required to be dynamically resolvable on ES.

By performing the ARB-style lookup for the suffixless function names we
can also support ES3 compatible contexts on desktop GL.

This also fixes the problem of picking up the APPLE extension functions
instead of ARB when both are available. vaoFuncsType was set to ARB correctly
but the helper has to take the preference of ARB into account too.

Task-number: QTBUG-38168
Change-Id: If7402320e8f96891017674f3c43bc57c4b5d29f3
Reviewed-by: Sean Harmer <sean.harmer@kdab.com>