Add a command line option to load Blender without a window frame.
Currently this is only used on Wayland, enabled for WITH_UI_TESTS since
attempting to load LIBDECOR caused the tests to crash on start on Fedora.
For tests there is no need to use LIBDECOR, so disable the window frame.
This can also be used by users who don't want to use the X11 fallback if
LIBDECOR can't be found.
Ref !147716
3349b97987