h264: make context_count unsigned

Removes the bogus but scary looking warning 'libavcodec/h264.c:4529:49:
warning: array subscript is below array bounds [-Warray-bounds]'.
jv: detect partial packets in the demuxer

Fixes fate-jv under valgrind which reports a different CRC for the last
frame from a partial read.
configure: avserver does not need $ldl

Loadable plugins were removed in d010e95f86089abe9a3d4d4a66ac8102312d28a4.
configure: do not link libraries against program-specific dependencies

Bug-Id: 635
mpeg12dec: do not add stereo3D side data to a non-existing frame

User data is usually coded before slice data. That means the frame
the user data belongs to is not available while parsing the user data.
The stereo3D side data has to use the same indirection over the private
context as pan scan information and A53 captions.

Bug-Id:632