x264.c: handle dts/pts from libx264 on X264_BUILD >= 83 onward

x264.c: handle dts/pts from libx264 on X264_BUILD >= 83 onward

libx264 gives dts-value, but we need to calculate initial delay, so dts doesn't
go < VLC_TS_0.

Tell how many frames are still left on libx264 buffer when we close encoder

Only set BLOCK_FLAG_TYPE_I on IDR-frames on X264_BUILD < 83 and only those
which have b_keyframe on X264_BUILD >= 83.