macosx: fix float-on-top behaviour with fullscreen

The issues were mostly noticable if you select both float-on-top
and fullscreen in video prefs.
transcode: encode buffer and flush encoder at the end when using threads
macosx: split fspanel from main menu nib to an extra nib which not being distributed as the window is being created entirely in code and the nib representation is used for development and layouting only