bstr.c doesn't really deserve its own directory, and compat had just
a few files, most of which may as well be in osdep. There isn't really
any justification for these extra directories, so get rid of them.
The compat/libav.h was empty - just delete it. We changed our approach
to API compatibility, and will likely not need it anymore.
Both X11 and Wayland support the same format for drag & drop operations
(text/uri-list), and the code for that was copied from x11_common.c to
wayland_common.c. Factor it out.