Get previews from H.264 content even if it's missing IDR frames (e.g., NZ TV & some...

Get previews from H.264 content even if it's missing IDR frames (e.g., NZ TV & some blu-rays):
 - Grab stream characteristics (IDRs or not, PCRs or not, RAPs or not) while we're reading to compute the duration rather than trying to guess with no information later.
 - Only wait for an IDR after a seek if we know the stream has IDRs. Even then, wait for at most 255 frames.
 - If the stream doesn't have IDRs tell scan (via a new flag in the title struct) so that it can read but discard a second's worth of frames to get the decoder in sync withe stream.
 - While we're trying to sync the decoder, ffmpeg will spit out dozens of useless error messages so suppress them.

git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@2071 b64f7644-9d1e-0410-96f1-a4d463321fa5