Changing output-background color code in partial-theming-watermark WF to a string

[snoesberger]

In the composite step of the partial-theming-watermark WF, the color code has to be defined as a string. Setting the value to a hex value like 0x000000FF leads to ffmpeg errors such as these:

...
INFO  | (EncoderEngine:495) - [Parsed_pad_1 @ 0x55744f9d0c80] Invalid 0xRRGGBB[AA] color string: '255'
INFO  | (EncoderEngine:495) - [Parsed_pad_1 @ 0x55744f9d0c80] Unable to parse option value "255" as color
INFO  | (EncoderEngine:495) - Error applying option 'color' to filter 'pad': Invalid argument
INFO  | (EncoderEngine:495) - Error opening output file /srv/opencast/workspace/mediapackage/909d5800-1d60-4de9-9efb-6e346e14f48a/1e6cac7f-7cbc-4445-9242-6d45abaa46d4/cc_ink_stamp_anim
ation__1080p__f7fc9977-21c3-4e02-a575-94b0433953cb-compound.mkv.
INFO  | (EncoderEngine:495) - Error opening output files: Invalid argument
WARN  | (EncoderEngine:245) - Error while encoding {video=/srv/opencast/workspace/mediapackage/909d5800-1d60-4de9-9efb-6e346e14f48a/1e6cac7f-7cbc-4445-9242-6d45abaa46d4/cc_ink_stamp_an
imation__1080p_.mp4}  using profile 'composite.http'
...

This pull request targets the legacy branch because watermark theming has probably not been working since the introduction of the YAML-WF.

How to test this patch

• activate theming for Opencast
• create a branding with watermark
• create a series that uses the branding with the watermark
• upload a video to that series
• check that the video is published with the watermark

Your pull request should…

• have a concise title
• close an accompanying issue if one exists
• be against the correct branch
• include migration scripts and documentation, if appropriate
• pass automated tests
• have a clean commit history
• does not incorrectly update the submodules
• have proper commit messages (title and body) for all commits
• explain why it needs to be merged into the legacy branch, if it is targeting the legacy branch
• include a release note if a feature, or supports a feature (ie, backend part of a frontend feature)