Set subbuffer fields as required

I've resolved a FIXME by following the docs here: https://gstreamer.freedesktop.org/documentation/gstreamer/gstbuffer.html?gi-language=c#gst_buffer_copy_region

The FIXME is on the selection of flags, in the end I've just gone with more flags just in case, but the main point is to ensure we overwrite timestamps, offsets, etc. when required.

I've done a very short run (600s) here: /fred/oz016/tdavies/projects/gstreamer_python_upgrade/testing/injection_tests/02_02_debug/16_02_sub_buffer_region_2
And in the new logs, the expected & actual values are identical.

There's a couple of other changes I think are safe. A redundant read/set of caps is removed (I've got a better idea of caps reading/setting now, very confident it's doing nothing). Plus some old debug & comments are removed.