Add dominant color background to Video blocks with poster images

[westonruter]

I realized there was a missed opportunity for adding a color dominant color background on Video blocks which have a poster assigned.

This currently only assigns the color when the video is uploaded to the Media Library, and the attachment post for the video itself is assigned a featured image which is the poster image.

• Support discovering the dominant color for posters that are assigned at the block level instead of the attachment level.
• Add a dominant_color_img_tag_add_dominant_color filter for the video case.