MacGui: fix picture sizing issue in presets where creating a preset using an HD sourc...

MacGui: fix picture sizing issue in presets where creating a preset using an HD source (ie. 960 x 544) and then using that preset on a smaller source (ie. dvd) would cause the smaller source to be upscaled to the larger size. Now preset is restricted to scale no larger than the current source regardless of size when it was created.

git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@1252 b64f7644-9d1e-0410-96f1-a4d463321fa5