Export: PNG with a transparent background (#784)

The renderer lays every slide background over white (as PowerPoint does),
so a PNG export was always opaque. A new transparentBackground option leaves
the slide background out (both the white underlay and the background fill),
so only what is on the slide is drawn over transparency. Shapes filled with
the background (useBgFill) still show it.

- render: RenderOpts::transparent_background
- engine: file.export {transparentBackground} (PNG only; other formats
  refuse it and write nothing), file.render {transparentBackground}
- UI: Transparent background checkbox in the Export dialog (PNG) and in
  File > Export > Change File Type > PNG; the web download honours it
- CLI: render --transparent

apps/deckcraft-cli/src/main.rs

@@ -244,11 +244,13 @@ fn render(args: &[String]) -> Result<(), String> {
     let mut all = false;
     let mut scale = 2.0;
     let mut edit = false;
+    let mut transparent = false;
     let mut out = None;
     let mut it = args.iter();
     while let Some(a) = it.next() {
         match a.as_str() {
             "--sample" => sample = true,
+            "--transparent" => transparent = true,
             "--slide" => slide = Some(it.next().and_then(|v| v.parse().ok()).ok_or("--slide needs a number")?),
             "--all" => all = true,
             "--edit" => edit = true,
@@ -281,7 +283,7 @@ fn render(args: &[String]) -> Result<(), String> {
         std::fs::create_dir_all(&out).map_err(|e| format!("{out}: {e}"))?;
     }
     for i in list {
-        let (png, w, h) = deckcraft_engine::cmd::file::render_png(&st.doc, i, scale, edit);
+        let (png, w, h) = deckcraft_engine::cmd::file::render_png_with(&st.doc, i, scale, edit, transparent);
         let path = if all { format!("{out}/slide-{:02}.png", i + 1) } else { out.clone() };
         std::fs::write(&path, png).map_err(|e| format!("{path}: {e}"))?;
         outln!("{path} ({w}×{h})");
@@ -680,6 +682,21 @@ mod tests {
         }
     }
 
+    /// `render --transparent` leaves the slide background out (#784): the corner is transparent.
+    #[test]
+    fn render_transparent_leaves_the_background_out() {
+        let dir = std::env::temp_dir().join(format!("deckcraft-cli-transparent-{}", std::process::id()));
+        std::fs::create_dir_all(&dir).unwrap();
+        let alpha = |path: &std::path::Path| deckcraft_engine::render::decode_image(&std::fs::read(path).unwrap()).unwrap().get_pixel(1, 1).0[3];
+        let opaque = dir.join("opaque.png");
+        render(&args(&["--sample", "--scale", "0.25", &opaque.to_string_lossy()])).unwrap();
+        assert_eq!(alpha(&opaque), 255);
+        let clear = dir.join("clear.png");
+        render(&args(&["--sample", "--transparent", "--scale", "0.25", &clear.to_string_lossy()])).unwrap();
+        assert_eq!(alpha(&clear), 0);
+        let _ = std::fs::remove_dir_all(&dir);
+    }
+
     /// `run --export OUT.gif --export-params JSON` and `convert IN OUT.gif` write animated GIFs.
     #[test]
     fn exports_an_animated_gif() {