Make the {{office}} macro generate 1024x768 images by default when viewing presentations

Right now the default is 800x600 and when clicking on the full screen button of the office macro, there's a small image in the center with a lot of black around.

Since our skins target a default screen estate of 1024x768 we should use the same default for the office macro.

Also, a good use case for the full screen is the ability to use that feature when a doing a presentation using a projector and a resolution of 1024x768 is a good standard definition (800x600 is too low a default).