QFont: Don't invalidate engine unless request has been changed

This makes QFont do a "light" detach when the font attributes data has been changed.
The new test clearly shows that the engine is now shared between
two font instances after changing the kerning attribute.

Change-Id: I59db822f459f02d111686dba7101b98e361fada9
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>
Reviewed-by: Lars Knoll <lars.knoll@digia.com>