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/fileio/encode.rs

@@ -417,8 +417,10 @@ fn encode_all_of(doc: &Document, format: &str, p: &Value) -> Result<Encoded> {
     vectorcraft_doc::cancel::check()?;
     // An image the renderer couldn't decode draws nothing: say so rather than write a picture
     // that silently lacks it.
-    if f.raster && vectorcraft_render::decode_failures() > failed {
-        enc.warnings.push("an image couldn't be decoded and is missing from the picture".into());
+    if f.raster
+        && let Some(w) = crate::cmd::rasterfx::undecoded_warning(failed)
+    {
+        enc.warnings.push(w.into());
     }
     // Formats that draw type: missing fonts come out in the fallback font (PDF and SVG say so
     // themselves: SVG only when it outlines or embeds fonts).