[3.0] subpicture: set regions to SDR by default

The caller of subpicture_region_New() can change it afterwards in the region format and the picture format. But default the values were undefined and might pick the colorimetry of the video.

The updater may reuse the video_format_t from the video which might be HDR.

Ref. #27877 (closed) and #26180 (closed)