Optimize the QMatrix4x4 after a conversion from SkMatrix44

This avoids using a General matrix for tile transformations which are
usually only translations in our case.

Change-Id: Ic947d7ef9b78de6d5d16ea718cc03255e38a8097
Reviewed-by: Michael Bruning <michael.bruning@digia.com>
Reviewed-by: Andras Becsi <andras.becsi@digia.com>