* Added a few comments. 
* Setting the currentMaster pointer to NULL each time we stop using it.  We should avoid using global pointers to access PPT specific information in DrawClient functions.
    

1. Let's create a drawing-page style for each main master slide.
2. Check what MUST be reused from style of the corresponding main master slide when creating a drawing-page style for a presentation slide.
3. Check what MUST be reused from style of the corresponding notes master slide when creating a drawing-page style for a notes slide.

Can't get the background color displayed in both kpresenter and ooimpress. Investigation required for kpresenter.  
The drawing-page style of the <presentation:notes> element in <master-page> element is the problem of ooimpress.  Their input filter is not using a drawing-page style there, which I think is not correct.