Resource Manager: Decode synchronously on the calling thread.

This commit changes synchronous decoding so that the calling thread is
the one which performs the decoding. Previously, decoding was done on
the job threads which was then waited on by an event on the calling
thread. The rationale for this design was to keep decoding on a single
code path, however this creates a problem for programs that would
prefer not to have any asynchronous job threads. In this case, these
synchronously decoded sounds would never get decoded because there
would not be any threads available to actually perform the decoding.

This commit enables the resource manager to be able to be used without
a job thread so long as asynchronous decoding and streaming are not
used. This scenario could be useful for programs that want to pre-load
all of their sounds at load time and save some system resources by not
incurring the overhead of an additional unnecessary thread.