x11_common: get hack for Motif Window Manager out of the way

Disable the hack by default, because it's interfering with some more
modern WMs. MWM (Motif Window Manager) is probably pretty dead, but
we still allow enabling the old hack with "--fstype=mwm_hack".

See github issue #179.
video: redo hw decoding initialization, add --hwdec=auto

Change how the HW decoding stuff is organized, the way it's initialized
in particular. Instead of duplicating the list of supported codecs for
hwaccel decoders, add a probe function which allows each decoder to
report whether it supports a given codec.

Add an "auto" choice to the --hwdec option, which automatically enables
hardware decoding if libavcodec and/or the VO supports it.

What mpv prints on the terminal changes a bit. Now it will just print
a single line whether hw decoding is used or not (and nothing at all if
no hw decoding at all was requested). The pretty violent fallback from
hw decoding to software decoding is still quite verbose and evil-looking
though.