Implement an h.264 decoder using Apple's VideoToolbox framework.
Based on gstreamer's vtdec because there's very little documentation
for this API beyond the framework's header file.
Reuse the MOZ_APPLEMEDIA define from the mp3 decoder.
AAC decoding is stubbed to use the BlankDecoderModule's 440A tone.
Provide a stub header for the VideoToolbox framework and load
it dynamically at run time so we can build and run on versions
prior to MacOS X 10.8 where it's not available.
Dynamically load the CoreMedia framework as well, but build
against its headers since it's available on MacOS X 10.7 or
later, which is our minimum build version.