More bugfixes:

- In SyncGPU, use GpuRunningState instead of EmuRunningState.  We want
to know if the GPU thread is running, so it's the right one to use, and
EmuRunningState was screwing things up by being true when the emulator
was not running (which is stupid, but...).

- Signal s_video_buffer_cond when RunGpuLoop exits to avoid a possible
hang when the emulator is stopped in the middle of GPU processing.