Avoid GCC's macro arg concat extension with IDE parsing
Don't assume the default mmdevapi device is in the collection
Don't define HAVE_DYNLOAD with the IDE parser
Store the old-style backend funcs in the wrapper
Remove some unneeded code and unnecessary macros in the OpenSL backend
Add another directory to .gitignore
Use a separate struct for tracking active sources
Store some source mixing parameters in the active source struct
Select the mixer when setting the mixer-specific parameters
Use a union to combine HRTF and non-HRTF mixer params
Keep track of the mix count

The purpose of this is to provide a safe way to be able to "swap" resources
used by the mixer from other threads without the need to block the mixer, as
well as a way to track when mixes have occurred. The idea is two-fold:

It provides a way to safely swap resources. If the mixer were to (atomically)
get a reference to an object to access it from, another thread would be able
allocate and prepare a new object then swap the reference to it with the stored
one. The other thread would then be able to wait until (count&1) is clear,
indicating the mixer is not running, before safely freeing the old object for
the mixer to use the new one.

It also provides a way to tell if the mixer has run. With this, a thread would
be able to read multiple values, which could be altered by the mixer, without
requiring a mixer lock. Comparing the before and after counts for inequality
would signify if the mixer has (started to) run, indicating the values may be
out of sync and should try getting them again. Of course, it will still need
something like a RWLock to ensure another (non-mixer) thread doesn't try to
write to the values at the same time.

Note that because of the possibility of overflow, the counter is not reliable
as an absolute count.