Fix build with dynamic GL.

Change-Id: I9dafdea8745e680a20c162496f3b5089b38037de
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>
Introduce structs for dynamically resolved functions per DLL.

Preparing the adding of the iconic preview functions; the current
approach of having one qt_ .. function does not scale well.

Task-number: QTBUG-36730
Change-Id: Id70b9b13b8cd09d4d9c2830e9d6d0a413fd812a3
Reviewed-by: Joerg Bornemann <joerg.bornemann@digia.com>