Envelopes: Anti-Alias and Preserve Shape Using change how distorted images draw

Envelope Options stored Anti-Alias and Preserve Shape Using, but nothing that
draws an image read them: a distorted image was always smoothed and always
cut into clip groups.

- With Anti-Alias off, each piece of a distorted image is marked pixelated
  (saved as `pixelated`). The renderer draws it with nearest-neighbor
  sampling and without mips, SVG export writes `image-rendering: pixelated`,
  and PDF export writes it without /Interpolate.
- With Preserve Shape Using Transparency, each piece is the image with a
  white opacity mask in the piece's shape instead of a clip group. The
  renderer paints an image whose mask is one plain white path straight into
  that path, as it does for an image in a clip group, when the path lies
  inside the image's frame.

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

crates/render/src/renderer.rs

@@ -1,4 +1,4 @@
-use super::blobs::{blob_key, blob_pixmap, image_mip, lookup, mask_value, mip_level, plain_image};
+use super::blobs::{blob_key, blob_pixmap, image_mip, lookup, mask_value, mip_level, plain_image, shape_masked_image, white_shape_mask};
 use super::clock::now;
 use super::frame::{
     ClipEntry, ClipPaintEntry, Frame, FrameStats, GeomEntry, MAX_INLINE_DEPTH, MISSING_INLINE_LOGGED, PlacedEntry, PlacedKey, PtrMap, SliceEntry,