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
Remove unneeded gui_update/reload_defaults calls