BitmapFilter fix

Would someone be able to have a look at the following patches?

I created the BitmapFilter idea some time ago and these have been successfully adopted for a number of features in LibreOffice. Some wonderful filters have been created since then, but because I didn’t really document what I was doing some of the filters don’t really follow the pattern. The code is really excellent, so I accept it was my fault for not explaining myself better. 

The first patch tries to remedy this by documenting the pattern. 

The second patch brings the code into line with the command pattern in BitmapFilter. 

If anyone would be willing to review this, it would be most appreciated :-)

Chris