avdec_h264: video decoder subframe support

This Merge request illustrates the work performed in gst-plugins-base!454 (merged)

It enables the support of slicing mode in avdec_h264

Here is a command line to test it:

$ gst-launch-1.0 -v videotestsrc num-buffers=80 ! video/x-raw,width=320,height=200 ! x264enc option-string="slices=4" ! h264parse ! video/x-h264,alignment=nal ! avdec_h264 ! videoconvert ! autovideosink

or

$ gst-launch-1.0 -v videotestsrc num-buffers=800 ! video/x-raw,width=320,height=200 ! openh264enc num-slices=4 slice-mode=n-slices ! h264parse ! video/x-h264,alignment=nal ! avdec_h264 ! autovideosink