Engine: Fixed a few mutexes that could be left in Waiting state.

This is essentially cosmetic. However, it could lead to an Assertion
when ptlib is compiled in DEBUG mode. This is ended better to make sure
a mutex is in "neutral" state when being destroyed and not waiting for
the corresponding Signal operation.
Add powerpcspe support
Fix crash with DEBUG=1 on mutex destruction

Completes commit a5151ef.
[windows] Fix encoding for video device name