add configuration value for VCL OpenGL backend

Change-Id: Ie23e31bec68e6f5ae3aab6ff5d6f19df5c2c9242
use configuration value for OpenGL backend

Change-Id: I18aa0a19a625b2a64d26adacaa139c9bf734dc9b
move that variable out of the constructor init list

We might delete it a few lines later again if we select the OpenGL one.

Change-Id: I08bac1b65bbae02f4f1dd149f484132f84dbc371
use constructor init list

Change-Id: I0344bdbd1e5643f8de1ce336caec21a3e789d4ca