Raster images: transparency off draws pixels opaque; TRANSPARENCY command

An image's transparency flag decides whether transparent pixels show what
is behind the image or draw in their stored colour: the image pass and PDF
plots now follow it (it was ignored, so every PNG alpha showed through).
TRANSPARENCY selects images and asks for the mode, defaulting to the first
image's current one.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

src/io/pdf_export.rs

@@ -242,8 +242,10 @@ fn emit_image(
     {
         return Err("Cannot plot bitmap: invalid coordinates, clip boundary, or opacity.".into());
     }
+    // The low bit (free: the pixel buffer is aligned) keeps the opaque copy
+    // of a picture apart from its transparent one.
     let key = (
-        std::sync::Arc::as_ptr(&image.pixels) as usize,
+        std::sync::Arc::as_ptr(&image.pixels) as usize | usize::from(!image.use_alpha),
         image.width,
         image.height,
     );
@@ -255,7 +257,14 @@ fn emit_image(
             doc.resources.xobjects.map.insert(
                 id.clone(),
                 printpdf::XObject::Image(printpdf::RawImage {
-                    pixels: printpdf::RawImageData::U8(image.pixels.as_ref().clone()),
+                    pixels: printpdf::RawImageData::U8(if image.use_alpha {
+                        image.pixels.as_ref().clone()
+                    } else {
+                        // Transparency off: every pixel opaque in its colour.
+                        let mut opaque = image.pixels.as_ref().clone();
+                        opaque.chunks_exact_mut(4).for_each(|px| px[3] = 255);
+                        opaque
+                    }),
                     width: image.width as usize,
                     height: image.height as usize,
                     data_format: printpdf::RawImageFormat::RGBA8,