It's not allowed to modify ctx->hw_device_ctx after avcodec_open2().
Therefore, we need to create the hw_frames_ctx ourselves from it, and from
the get_format() callback. It can be easily done with the
avcodec_get_hw_frames_parameters() helper.

The surface frame count is now known from the Create() function,
therefore, the semaphore can be directly initialized, removing the need
for the pool_sem_init dirty hack.