Pictures: compress pictures to a target resolution

Picture Format › Adjust gains Compress Pictures, with PowerPoint's
options: HD 330, Print 220, Web 150 or E-mail 96 ppi, apply to the
selected pictures or all, and delete cropped areas. The new
picture.compress command downsamples each PNG, JPEG, BMP, TIFF or WebP
picture to the pixels its shown size needs at that resolution (JPEG
stays JPEG at quality 85, the rest become PNG), cuts away cropped areas
when asked, and only replaces a picture when the result is smaller or a
crop was removed. Originals nothing else uses are dropped so the saved
file shrinks; one shared with a picture that wasn't compressed stays.
One undo step restores everything.

Co-authored-by: Cursor <cursoragent@cursor.com>

docs/gaps.md

@@ -186,7 +186,7 @@ Detail in [animation-parity.md](animation-parity.md). 10–16 h; the pane is a *
 ### Smaller missing rows
 Zoom (summary/section/slide, 6–10 h), reuse slides (2–4 h), laser pointer (1–2 h), transition
 sounds (1–2 h), artistic effects (6–10 h), remove background (6–10 h; needs a model or a classical
-matte), compress pictures (2–3 h), insert screenshot (2–3 h), record audio/screen and record slide
+matte), insert screenshot (2–3 h), record audio/screen and record slide
 show (8–12 h), compare presentations (6–10 h), password protection (4–6 h, ECMA-376 agile
 encryption), ink to shape/text (6–10 h), icons and 3-D models (8–14 h, needs an open icon set).