Rasterize, Flatten Transparency: commands that replace art with a rendering warn about an undecodable image

Object > Rasterize, Crop Image, Flatten Transparency, Expand
Appearance of raster effects and Vector Halftone draw the art with the
renderer and replace it with the result. When the art held an image
that could not be decoded, the renderer drew nothing for it, and the
result lacked the image with no message. A raster export of the same
art warns that "an image couldn't be decoded and is missing from the
picture".

These five commands now compare decode_failures before and after they
draw, as raster exports do, and their replies carry that text as
`warning`. They still make their result in one undo step. The app
shows a command's `warning` in the status bar: after a menu command,
after OK in a dialog with a preview, and after a tool runs a command
(Crop Image's Apply).

Docs: docs/mcp.md (Flatten Transparency, Expand Appearance, Document
Raster Effects Settings, Vector halftone).

crates/engine/src/cmd/rasterfx.rs

@@ -159,6 +159,12 @@ fn render(doc: &Document, nodes: Vec<Node>, region: Rect, scale: f64, anti_alias
     r.render_region_with(&tmp, region, scale, &RenderOptions { skip_templates: true, anti_alias, supersample: true, ..Default::default() })
 }
 
+/// The note for pixels drawn since [`vectorcraft_render::decode_failures`] stood at `failed`, when
+/// an image among them couldn't be decoded and so drew nothing; `None` otherwise.
+pub(crate) fn undecoded_warning(failed: u64) -> Option<&'static str> {
+    (vectorcraft_render::decode_failures() > failed).then_some("an image couldn't be decoded and is missing from the picture")
+}
+
 /// Pixels per point of raster effects rendered as images (the document's raster effects
 /// resolution).
 pub(crate) fn effects_scale(doc: &Document) -> f64 {