vo_null: don't reject hwaccel formats

This is not strictly needed anymore. (On the other hand, it's not really
possible to do hw decoding with vo_null, because the VO is still
responsible for opening the hw decoder API, but that's another story.)
input: improve some comments

I have the feeling some of these were written before everything was
changed all over again.
input: don't emit command when using multiple mouse buttons at once

This is for key bindings that use multiple mouse buttons at once. (Yes,
this is weird, but MPlayer always had this feature, and apparently
there are people using it!)

Before this commit, clicking another mouse button while still holding
the previous mouse button forced the command bound to the previous
mouse button to be emitted. This is usually needed to make sure the
input consumer (the player and the OSC) stays in sync with the actual
mouse button state. If there's no command sent, the OSC in particular
would think the button is still held down. However, sending the command
is undesired behavior if you want to use these multiple-key binds.

Solve this by emitting commands in this situation only if a key down
command was sent earlier. Since mouse button key bindings are normally
executed on key-up only, this happens with special commands like
script_dispatch only (used by the OSD to track mouse buttons, but
also used for other OSC bindings).

See github issue #390.