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).

docs/mcp.md

@@ -1482,7 +1482,9 @@ opacity masks or raster effects reach (clipped to those regions with `clipComple
 `preset` is `high`, `medium` (the default) or `low`; option keys (`balance` 0–100, `lineArtPpi`, `gradientPpi`,
 `textToOutlines`, `strokesToOutlines`, `clipComplexRegions`, `antiAlias`, `preserveAlpha`, `preserveOverprints`)
 override it, at the top level or in `options`. `balance: 0` rasterizes everything; lower balances rasterize groups
-that split into many regions. The result reports `{ids, vector, rasterized, options}`.
+that split into many regions. The result reports `{ids, vector, rasterized, options}`. When an image in the art
+can't be decoded, the image that replaces the art does not show it, and the result also carries `warning`: "an image
+couldn't be decoded and is missing from the picture", the note raster exports give.
 
 ```json
 {"name":"run_command","arguments":{"command":"object.flattenTransparency","params":{"ids":[12,15],"preset":"high"}}}
@@ -1605,7 +1607,8 @@ document's raster effects resolution (shadows and outer glows under the art; a b
 the object with the image). Type with a geometry effect (Outline Object included) or with fills and strokes of its own
 is outlined; type that has no fills or strokes of its own and whose only effects are shadows or outer glows stays
 type, with the image under it. A group's or layer's own fills and strokes become objects among its members, and its
-members are expanded too.
+members are expanded too. The result is `{ids}`; when an image in the art can't be decoded, the images made from
+raster effects do not show it, and the result also carries the `warning` that Flatten Transparency gives.
 
 ```json
 {"name":"run_command","arguments":{"command":"effect.expandAppearance","params":{"ids":[12]}}}
@@ -2520,6 +2523,8 @@ in one undo step. PDF export renders the raster effects of every kind of object
 images, symbol instances, live objects, and those in opacity masks, symbols and pattern tiles): shadows and outer glows
 go in an image under the untouched vector art, other effects replace the object with its image, and a path whose
 fills or strokes carry effects is written as one piece per fill and stroke, so only the affected ones become images.
+When an image in the art can't be decoded, `object.rasterize` and `object.cropImage` (without `trim`) make their image
+without it, and their results carry the `warning` that Flatten Transparency gives.
 
 ```json
 {"name":"run_command","arguments":{"command":"document.rasterEffectsSettings","params":{"resolution":"high","background":"white","addAround":36}}}
@@ -3612,7 +3617,9 @@ effects keep their colours.
 ## Vector halftone
 
 `object.vectorHalftone` turns the selection into vector dots, lines or shapes sized by its
-tone (mono, or CMYK screens multiplied over each other), clipped to the art's outline by default:
+tone (mono, or CMYK screens multiplied over each other), clipped to the art's outline by default. An image in the
+art that can't be decoded is sampled as blank (no dots, or full dots with `invert`), and the result carries the
+`warning` that Flatten Transparency gives:
 
 ```json
 {"name":"run_command","arguments":{"command":"object.vectorHalftone","params":{"shape":"circle","frequency":25,"angle":45,"mode":"cmyk"}}}