subpicture: initialize the region picture with the region format

The region format has its own copy of the palette, whereas the source may have a temporary pointer.

This is working most of the time as the region format is usually used to get the palette of a region. But this makes the code potentially crashing if just the region picture is used.

The same problem exists in 3.0. But in 4.0 the picture palette is more prioritary than the region one since 1b5faa0f.