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.
The values are expected to be in the 0.0-1.0 range (as indicated by the
comment), and other parts of Dolphin also expect it to be in that range
since the "full" axis has a -1.0 to 1.0 range. However, this is not
always the case and fvalue can end up being outside of the range. This
clamps fvalue to always be in the 0.0 and 1.0 range.
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.
Léo Lam