mpegvideo: Use the current_picture pts

The picture slot can be recycled by select_input_picture and
only current_picture is populated with the valid pts.

Unbreak timestamps when in cbr mode.

Signed-off-by: Luca Barbato <lu_zero@gentoo.org>
(cherry picked from commit 1c7b71a5bdb88ebb69734100405bbb5441b871e8)
Signed-off-by: Anton Khirnov <anton@khirnov.net>

Conflicts:
	libavcodec/mpegvideo_enc.c