Switch buf->key to buf->frametype which is a bitmask telling us what type of frame...

Switch buf->key to buf->frametype which is a bitmask telling us what type of frame we are dealing with.
This doesn't change any functionality, but I need to be able to distinguish between x264 IDR and I frames for the upcoming matroska muxer.
This also has the side effect of making the code a little easier to read and maintain.

git-svn-id: svn://localhost/HandBrake/trunk@623 b64f7644-9d1e-0410-96f1-a4d463321fa5