input: set mouse area by default for all input

Otherwise, mouse button bindings added by mp.add_key_binding() would be
ignored.

It's possible that this "breaks" some older scripts using undocumented
Lua script functions, but it should be safe otherwise.

Fixes #1283.
x11: fix crash with --wid=0 and vo_xv/vo_x11

But seriously, don't use --wid=0, don't use vo_xv, and _especially_
don't use vo_x11.

Fixes #1284.