This implements basic tests for functionality related to loading and
processing external album covers, both on the scanning size, and on the
display side.
Files that match the `CoverArtPriority` setting will now be considered
eligible only if their extensions are of an 'image/*' MIME type (e.g.
'.png' for 'image/png', '.jpg' for 'image/jpeg'). This prevents matching
files that will likely not be valid during display.
In addition to the above, code for returning the cover image file from
scanned data will also check against the MIME type for the path stored,
instead of attempting to re-trace `CoverArtPriority` matches. This
simplifies the code and bypasses a number of edge-cases related to
inconsistent matching.
This commit adds support for loading cover art from media file
directories, according to configured filename priorities (of which an
additional, special choice of `embedded` is given).
Cover art paths are resolved during scanning and stored in the database
as part of the `album.cover_art_path` column; if embedded cover art is
matched, this will default to the path of the media file itself, and if
no cover art is matched at all.
Similarly, the `album.cover_art_id` column will default to a reference
to `media_file.id` if embedded cover art is wanted, but if an external
cover art file is matched, this will instead be set to a reference to
the `album.id` value itself, prefixed with the `al-` constant.
Stored cover art paths are once again resolved and matched against
configuration when covers are requested; that is, any change in
configuration between scanning and requesting cover art may not return
correct data until a re-scan is complete.
Tests will be added in future commits.
