Xlib supports many mouse buttons, though there are 9 standard buttons, and they aren't arranged like other mouse APIs. Using only 5 buttons was preventing the use of buttons besides left/right/middle click and the scroll wheel. Here's what all the standard buttons are:
1. left button
2. middle button (pressing the scroll wheel)
3. right button
4. turn scroll wheel up
5. turn scroll wheel down
6. push scroll wheel left
7. push scroll wheel right
8. 4th button (aka browser backward button)
9. 5th button (aka browser forward button)
The remaining button indices are non-standard and device-specific, and technically far more than 32 are supported, but this seems like a reasonable limit to avoid cluttering the list with tons of useless mouse buttons. What mouse has more than 32 buttons anyways?
The SDL backend crashes when you close a joystick after SDL_Quit has
been called. Some backends don't need to be shutdown and
re-initialized everytime, we can just ask to enumerate devices again.
This makes the device ID assigning code common to all backends, by
moving it to AddDevice() instead of copy-pasting or replicating
the logic in the backends.
Also, to prepare for hotplugging, instead of relying on a name usage
count, the new ID assigning system always starts from ID 0 and tries
to assign the first ID that is not used.
Small cleanup by using std::shared_ptr and getting rid of
ciface.Devices() which just returned the m_devices (which defeats the
point of making m_devices protected).
Incidentally, this should make the code safer when we have
different threads accessing devices in the future (for hotplug?).
A lot of code use Device references directly so there is
no easy way to remove FindDevice() and make those unique_ptrs.
Previously, the devices vector would be passed to all backends. They
would then manually push_back to it to add new devices. This was fine
but caused issues when trying to add synchronisation.
Instead, backends now call AddDevice() to fill m_devices so that it is
not accessible from the outside.
