core: Define specific image formats to be stored in the clipboard manager

And order those preferences in order of lossiness (jpeg < png < bmp). This
avoids us from prefering other formats that are not useful or widely
recognized.

https://gitlab.gnome.org/GNOME/mutter/merge_requests/939

src/core/meta-clipboard-manager.c

@@ -32,7 +32,9 @@ static struct {
   const char *mimetype_glob;
   ssize_t max_transfer_size;
 } supported_mimetypes[] = {
-  { "image/*",                  MAX_IMAGE_SIZE },
+  { "image/jpeg",               MAX_IMAGE_SIZE },
+  { "image/png",                MAX_IMAGE_SIZE },
+  { "image/bmp",                MAX_IMAGE_SIZE },
   { "text/plain",               MAX_TEXT_SIZE },
   { "text/plain;charset=utf-8", MAX_TEXT_SIZE },
 };