clip input motion vectors

clip input motion vectors
be somewhat more tollerant on invalid input
return INT_MAX instead of -1 for unuseable mv/mb types as that ensures nicely that they arent used
initalize limits earlier for b frames
a few more asserts to check for out of picture vectors

git-svn-id: file:///var/local/repositories/ffmpeg/trunk@3213 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b