Fix raw parsing race condition
We get the initial dt_image_t from the database and then read the raw. The settings that come from the raw but not the database are not there. In the past we've papered over this by adding things to the db that have no business being there (e.g., raw_black and raw_maximum). Instead we can swap the correct dt_image_t into the pipeline at the same point we introduce the buffer itself into the pipe. Let the metadata flow together with the data.
Don't keep thumbs if we've dropped the metadata
Since we sometimes need some of the raw metadata from dt_image_t to interpret "thumbnails" (really DT_MIPMAP_F images) we drop them when we drop the metadata. This should be mostly harmless for performance as a lot more metadata should fit than image data, so evictions in the metadata cache should be much rarer.
Run reload_defaults only once after img is loaded
Rework when reload_defaults gets called