add C++ header safeguards so we can use OMXIL from C++ easily.

Signed-off-by: Ithamar R. Adema <ithamar@upgrade-android.com>
omx_base_component: bail out if we're destroyed before we noticed.

Signed-off-by: Ithamar R. Adema <ithamar@upgrade-android.com>
omx_base_port: increase default # buffers from 2 to 4.