plug-ins: fix dds problems found in #12660
@cmyk.student Maybe you could have a look at these commits, to make sure I didn't do anything too crazy.
This should both improve our conformity with other DDS readers by exporting more closely to DDS standard; and add support for reading these non-standard types the way we wrote them in the past.
Note: we should probably add examples of these image to our tests to make sure we don't loose support in the future.