Images: SVG and EPS export write colorized grayscale and 1-bit images in their fill color

SVG export ignored an image's colorize paint and wrote the image's own
black, white and gray pixels. EPS export flattened every colorized image
into a process-color raster, so a spot color lost its plate. Both now
write the image in the fill color, with the coverage the canvas and PDF
export use (darkness times alpha), so white and transparent pixels take
no color.

- SVG writes a coverage PNG as a mask over a rectangle in the color,
  shared between repeated images while each keeps its own color. Linked
  mode writes the coverage PNG as a file, and SVG Tiny 1.2 writes a PNG
  in the color. A colorized image whose pixels are missing, or whose
  grayscale pixels can't be decoded, is left out with a warning.
- EPS writes fully covered or uncovered pixels as an imagemask in the
  color's process or Separation ink (Registration on every plate), and a
  grayscale image as ink samples scaled by coverage. Level 3 masks its
  uncovered pixels out and Level 2 writes them white; gray midtones are
  written over white, with the existing warnings.
- EPS flattening no longer counts a solid colorize paint on a grayscale
  or 1-bit image as transparency by itself. Opacity, blending modes,
  opacity masks, effects and overlapping transparent art still flatten
  it, and so does a colorize paint the writer can't draw.

The svg crate's tests turn on resvg's raster-images feature to render
the embedded PNGs.

Docs: docs/mcp.md, docs/target-app-parity.md.

docs/mcp.md

@@ -585,6 +585,7 @@ font whose licence (OS/2 `fsType`) forbids subsetting is embedded whole, one tha
 with a warning. Type names its faces by numeric `font-weight` (600 for Semibold and for a Japanese face's W6, 300 for Light, `bold` for 700),
 embedded or not. `svgz` takes the same options and writes the SVG gzipped (`.svgz` files also open,
 place and paste). `run_command document.save {path: "x.svg", svg: {…}}` saves as SVG (or `.svgz`).
+Colorized grayscale and 1-bit images export in their fill color; linked mode writes a coverage PNG, and Tiny 1.2 writes a colorized PNG.
 
 Symbols export as one `<symbol>` with a `<use>` per instance. An instance the def can't stand for is written as its
 own art: one stained by its fill; one scaled while the symbol has strokes (their weight doesn't scale), or rotated or
@@ -4038,6 +4039,7 @@ is accepted); spot colours are Separation colour spaces, and overprinting fills
 `overprints: "discard"`. RGB documents are written in CMYK unless `cmykPostScript: false`. When the file would hold an
 image that can't be decoded, the image is left out, and `warnings` carries "an image couldn't be decoded and is missing
 from the picture" once for all such images.
+Colorized grayscale and 1-bit images use their process or spot ink; the flattener leaves them alone unless they have opacity, a blending mode, an opacity mask or effects, or overlap other transparent art. Binary coverage leaves underlying art untouched. For a grayscale image, Level 3 leaves white (zero-coverage) pixels out, Level 2 writes them white, and gray midtones are written over white at both levels.
 
 PostScript has no transparency: it is flattened first with `flattenerPreset` (`high`, `medium` (default), `low` or a
 saved preset, see `flattener.presets.list`) and `flattener: {…}` options over it (those of