image: ensure picture is released from client

Ensure the client calling the export to block_t is also responsible for releasing the picture it exports.

Previous commits were adding the release to match behaviour of the converter before refactoring everything, but it was a mistake and led to issues with other parts of the pipeline, in particular the transcoder, and some clients were more adapted to the converter path + hold of original rather than the non-conversion path, which would then have been simpler to do from the start.

This MR adds the necessary hold and release to remove those from the encoders and the clients, and is tested against the unmerged libvlc/media.c test for thumbnailing too.

Revert "png: fix picture release on encoder" This reverts commit d4c93a29.

Revert "jpeg: fix picture release on encoder" This reverts commit cccc085e.

Revert "avcodec: encoder: fix picture release" This reverts commit 5a145f34.

Revert "codec: vpx: fix picture release on encoder" This reverts commit 5dde7db3.

Revert "image: fix picture leak in case of error" This reverts commit 1bf09fe8.

Revert "misc: thumbnailer: release picture when interrupted" This reverts commit 004b9900.