OpenGL, PlatformSupport: use QStringRef to optimize memory allocation

Replace substring functions that return QString with
corresponding functions that return QStringRef where
it's possible.

Create QString from QStringRef only where necessary.

While touching the code, also port loops to C++11 style.

Change-Id: Ib94d29b6ecec1cdab2604d0277a743e8b340be5e
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>