PDF: colorized images count as transparency for PDF 1.3 and PDF/X

A colorized grayscale or 1-bit image draws its white pixels transparent and is written to
PDF with the image as a soft mask. Node::shows_transparency did not count it, so a PDF 1.3,
PDF/X-1a or PDF/X-3 export whose only transparency was a colorized image was not flattened
and kept the soft mask. A legacy .ai save also wrote such an image as opaque.

- shows_transparency counts a visible image with a colorize paint.
- The legacy .ai writer flattens or leaves out colorized images like other see-through images.

crates/doc/src/node/ops.rs

@@ -683,8 +683,8 @@ impl Node {
 
 impl Node {
     /// Whether this object shows any transparency, itself or anything inside it: object, fill or
-    /// stroke opacity below 100%, a blend mode, an opacity mask, effects or transparent gradient
-    /// stops. Hidden objects show none.
+    /// stroke opacity below 100%, a blend mode, an opacity mask, effects, transparent gradient
+    /// stops or a colorized image. Hidden objects show none.
     pub fn shows_transparency(&self) -> bool {
         use vectorcraft_color::Paint;
         let see_through = |p: &Paint| matches!(p, Paint::Gradient(g) if g.gradient.stops.iter().any(|s| s.opacity < 1.0));
@@ -693,6 +693,7 @@ impl Node {
             && (self.opacity < 1.0
                 || self.blend != BlendMode::Normal
                 || self.extras().mask.as_ref().is_some_and(|m| !m.disabled)
+                || (matches!(self.kind, NodeKind::Image(_)) && self.extras().colorize.is_some())
                 || effects(&self.appearance.effects)
                 || self
                     .appearance