Hand the drag ghost to the renderer through a file, so it draws

The bitmap under the cursor was a solid red rectangle. Slint's drag
overlay uploads the image as a texture, draws it and drops the texture in
one call; with the wgpu FemtoVG renderer the drop is immediate and the
draw is deferred to the flush, so the frame binds femtovg's placeholder —
which is red. An image with a cache key survives in the texture cache
until after the flush, and only a path gives one. So the composite goes
to the data directory's scratch as a PNG and comes back through
load_from_path; one file per drag, removed when the drag ends. A
workaround for Slint 1.17.1, written up as one beside the code.

ui/dr-ui/Cargo.toml

@@ -53,6 +53,10 @@ dr-face = { workspace = true, features = ["inference"] }
 # runtime file; the build stays C-free either way (docs/inference.md §3).
 dr-inference-engine = { workspace = true, features = ["native"] }
 dr-thumbs.workspace = true
+# For the drag ghost only: the composite under the cursor has to reach the
+# renderer through a file, and the fan of thumbnails needs alpha, which the
+# thumbnail store's JPEG cannot carry. See `collections_ui::drag_image_via_file`.
+png = "0.18"
 # The library module writes scan results straight into the catalog, so it
 # needs the same SQLite types dr-catalog exposes.
 rusqlite.workspace = true