[NEED TESTING FROM USERS!] A HUGE optimization for the merger framework

Now the merger deals much more carefully with the layer's extent and
doesn't write data if the area in question is empty! That is really
important when working with transformation and transparency masks.

What needs to be tested really thoroughly:

1) Using Move Tool

* on usual layers
* on group layers
* on layers with transparency masks
* on layers with transformation masks

2) Updates when using Transformation Masks

3) Updates when using Transparency Masks


CCMAIL:kimageshop@kde.org