CanvasContext, CanvasGlCommandQueue: use prefix ++ for iterator

because it's useful for non-primitive types

Change-Id: Ic0a53b24148c5815b881d72eecadcaa4ca1ed3e5
Reviewed-by: Tomi Korpipää <tomi.korpipaa@theqtcompany.com>
Reviewed-by: Pasi Keränen <pasi.keranen@theqtcompany.com>