screenshot: add format specifiers to get file directory path

Useful if you want to put the screenshot into the same directory as the
file that is being played.
player: strip 'file://' from filenames on playback start

This fixes two things:

1. Dropping files on the VO window will auto-load subtitles (since most
   drag & drop code prefixes the filenames with 'file://', and the
   subtitle auto-load code considers 'file://' non-local)
2. Fix behavior of the %x screenshot filename template (similar problem)

One could force all that code to special-case 'file://' URLs, but just
replacing the filename on playback start is simpler.
wayland: fix crash when initialization fails

On X11, if no wayland compositor is running, wl_list_init() will never
be called. This will cause destroy_display() to segfault when trying to
iterate over the list.