If the remove_connection call of obs_encoder_stop_internal took too
long, obs_encoder_destroy could get called before that function
completed, causing a race condition.
Allows the ability to encode by passing NV12 textures. This uses a
separate thread for texture-based encoders with a small queue of
textures. An output texture with a keyed mutex shared texture is locked
between OBS and each encoder. A new encoder callback and capability
flag is used to encode with textures.
