write 16 bit PNG files even when there is no float buffer,
the existence of a float buffer can depend on the image being used in the compositor for example, so better give the user what they expect - 16bit png if its selected from the UI.


git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@53804 954f8c5b-7b00-dc11-b283-0030488c597c