dt_imageio_open_rawspeed(): always calculate black areas

In CR2's case, we might now read separate levels from the MakerNotes,
and thus never sample the (more correct) Black areas,
because separate black levels are no longer unset.

I'm surprised this wasn't being done in the first place,
handling of black areas seems to be a somewhat too ugly...

src/imageio/imageio_rawspeed.cc

@@ -208,7 +208,7 @@ dt_imageio_retval_t dt_imageio_open_rawspeed(dt_image_t *img,
     img->raw_black_level = r->blackLevel;
     img->raw_white_point = r->whitePoint;
 
-    if(r->blackLevelSeparate[0] == -1
+    if(!r->blackAreas.empty() || r->blackLevelSeparate[0] == -1
        || r->blackLevelSeparate[1] == -1
        || r->blackLevelSeparate[2] == -1
        || r->blackLevelSeparate[3] == -1)