vo/x11_common: don't require a working input method

Normally, we need this for Xutf8LookupString(). But we can just fall
back to XLookupString(). In fact, the code for this was already there,
the code was just never tested and was actually crashing when active
(see commit 2115c4a).
command: use a list of potential deinterlacer filters

Instead of hardcoding a single filter. This might be helpful for
modeling the vaapi deinterlacer as a video filter. The idea is that a
software deinterlacer would be tried first, and if that fails (because
vaapi hardware decoding uses HW surfaces, which a software deinterlacer
does not accept), the vaapi filter would be tried.