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/src/library.rs

@@ -4340,6 +4340,15 @@ pub fn inference_cache_dir() -> PathBuf {
     data_root().join("inference")
 }
 
+/// Somewhere to put a file that only needs to exist for a moment.
+///
+/// Under the data root rather than `std::env::temp_dir()`, which on Android
+/// names a directory the app cannot write to. Nothing here survives a
+/// launch on purpose: whoever writes into it deletes what they wrote.
+pub fn scratch_dir() -> PathBuf {
+    data_root().join("scratch")
+}
+
 /// The detector and embedder files, if both are present — and the eye
 /// models beside them, if those are.
 ///