hevc: templatize pred_planar

Signed-off-by: Anton Khirnov <anton@khirnov.net>
hevc: templatize intra_pred
hevc/intra_pred: optimize EXTEND_()*

Process 4 pixels at once.

Signed-off-by: Anton Khirnov <anton@khirnov.net>
hevc/intra_pred: drop unnecessary conditions in loops

Signed-off-by: Anton Khirnov <anton@khirnov.net>
hevc/intra_pred: simplify neighboring sample derivation
avconv: do not use poorly defined and undocumented AVStream.pts

Use OutputStream.last_mux_dts as an approximation of 'current' output
time instead.
mux: drop one of the hacks comprising compute_pkt_fields2()

All encoders should output proper timestamps now.
lavf: deprecate now unused AVStream.pts