For presentation window, if current size is bigger than default, don't...

For presentation window, if current size is bigger than default, don't reinitialize to default size when new
presentation is loaded. However, if window size is smaller than the default, reinit the window to default size.
This prevents the presentation window from becoming progressively smaller as presentation get loaded.

git-svn-id: http://bigbluebutton.googlecode.com/svn/trunk@3959 af16638f-c34d-0410-8cfa-b39d5352b314