TextureMapperGL must take hiDPI transform into account

Since TextureMapperGL does its own native rendering it must itself
apply the all transforms QPainter would normally do behind the scenes.

Task-number: QTBUG-36508
Change-Id: Id4a0ee64ff7ac5c5cb51e45ffd2e4e25ed7fce74
Reviewed-by: Michael Bruning <michael.bruning@digia.com>
Reviewed-by: Jocelyn Turcotte <jocelyn.turcotte@digia.com>