So that it's not needed to set up GST_PLUGIN_PATH accordingly or install
the plugin first before running the examples.

We round down to the previous chunk boundary, but then we also have to
add the difference between the offset of that chunk to the requested
offset to the size of the buffer that is pulled from upstream. Otherwise
there will be an unneeded short read, which could cause downstream to
EOS too early.

The only None ranks now are:
- cdgdec/cdgparse: Primary
- rav1enc: Primary
- reqwesthttpsrc: Marginal
- s3sink/s3src: Primary
- mccenc/mccparse: Primary
- sccenc/sccparse: Primary

All other ranks are set to None until the elements are ready to replace
the existing C elements, or are otherwise production-ready and of a
element kind that should be autoplugged in one way or another.

For example the sodium elements are production-ready but can't be
autoplugged and as such should stay at a None rank.

Rank is now an enum rather than a u32

With some changes by Sebastian Dröge <sebastian@centricular.com>